I found this paper that may be useful for you. It shows best angles at various lower latitudes. I hope this is useful.

For exactly estimating the tilt angle you need precise set of data regarding the solar radiation flux density received by the Earth for the studied geographical location, which is the result of two components: direct (or beam) solar radiation flux and diffuse solar radiation flux.

Nevertheless, in technical literature, there are some practical recommendations concerning the fixed optimum tilt angle. For example 0.9*latitude or just equal to latitude. In addition, the simplest way is to mount your solar panels at a fixed tilt, but because the sun is higher in the summer and lower in the winter, more energy during the whole year could be captured by adjusting the tilt of the panels according to the season. In this case, one of rules of thumb is latitude + 15 for winter and latitude - 15 for summer. In all cases the collector should face the equator.

In addition, I recommend you the following articles:

[1] S.Sh. Soulayman, On the optimum tilt of solar absorber plates, Renewable Energy, Volume 1, Issues 3–4, 1991, Pages 551-554, ISSN 0960-1481, http://dx.doi.org/10.1016/0960-1481(91)90070-6

[2] Camelia Stanciu, Dorin Stanciu, Optimum tilt angle for flat plate collectors all over the World – A declination dependence formula and comparisons of three solar radiation models, Energy Conversion and Management, Volume 81, May 2014, Pages 133-143, ISSN 0196-8904, http://dx.doi.org/10.1016/j.enconman.2014.02.016

[3] Milan Despotovic, Vladimir Nedic, Comparison of optimum tilt angles of solar collectors determined at yearly, seasonal and monthly levels, Energy Conversion and Management, Volume 97, June 2015, Pages 121-131, ISSN 0196-8904, http://dx.doi.org/10.1016/j.enconman.2015.03.054

As the saying of Milan Despotovic and others, it is suggested the following tilt angles: ∅ + 15° for optimum and ∅ − 15° (for summer and winter) or ∅ for optimum power production. But it can be change according to many factor.

For example, in our study (DETERMINATION OF SOLAR COLLECTOR OPTIMUM TILT ANGLE FOR ANKARA AND DISTRICTS):

In Ankara, Turkey, fixed solar collectors are usually placed at slope of 40 degree. In this study, annual optimum tilt angle for Ankara province and districts was calculated as 34° however the optimum angles during the year were calculated between 1° and 67°. The results of this study are compared with similar studies in the literature they were found to be consistent with the results of similar studies. Also, the tilt angle was calculated for the six months periods as 15° and 56°, for seasonal periods (winter, spring, summer, autumn) as 62°, 23°, 6° and 49°, respectively and solar radiation for optimum angles of inclination were found.

Thank you all.

The problem is that our collector lies within the tropics i.e. in summer, June 21st the sun is at latitude 23.5 deg north and so tilting towards the south would not be beneficial as the sun will be behind the collector. Clearly the rule of thumb ( latitude angle + 15) may not be applicable in this case.

In the following paper you can find the periods when the collector should be oriented toward equator and when in opposite direction, and monthly optimum tilt angles.

The exact angle does not matter much, so anything between 1° and 15° would be fine, To do more fine tuning, you would need to run a simulation. As a general rule, if the climate is very cloudy,  go for 1° to 5°. If rather very sunny, go for tilt=latitude. 0° is not recommended because of the risk of rain accumulation.

Do you mean longitudinal or latitudinal tilt? Assuming the latter, M Gueymard's answer is very practical and probably one that would be adopted by most installers. However if you have a pitched roof with the apex generally aimed north/south, you have to decide which side of the roof to put the panels  and you're largely stuck with the angle the roof is built to. If the likelihood of cloud is evenly split between morning and afternoon, place the panels in the east face as it's generally cooler and panels  will give a little more energy when they're cool, in the morning.

The general practice is to keep tilt angle same as latitude of the location. However, during winter it could be  Latitude +10 to 15 degree and for summer Latitude - 10 to 15 degree.

For precise result, optimum tilt angle may be calculated by referring to the expression available in few research papers as quoted by others

check out my paper, may be helpful

https://www.researchgate.net/publication/283051726_A_comprehensive_solar_angles_simulation_and_calculation_using_Matlab

Article A comprehensive solar angles simulation and calculation using Matlab