Possibly zero.

If you are using Bt as a natural insecticide then it works by infecting caterpillars. The bacteria grow inside the caterpillar. The caterpillar changes behavior and climbs the plant. At some point it dies, and the bacteria are released to ooze down the plant to infect other caterpillars (or dry and blow off in the wind). Deltamethrin will kill the caterpillar before this process has a chance to work.

Possibly the answer is the standard field rate of deltamethrin.

Is this as a resistance management strategy, or if Bt does not work why not just use deltamethrin?

Dear Timothy,

The idea is not using the full recommended rate of deltamethrin to reduce environmental impact,

A sub lethal dose of deltamethrin could stress the caterpillars, increasing the effect of Bacillus thuringiensis..  

Why do you want to use them together?, using sub-lethal doses is always a good precursor for resistance development.  You also need to know is they are  compatible?..which pest and on what crop?, which product of Deltamethrin and what concentration? I used Deltaplan 12.5 EC at a rate of 0.0375 g/L for bioassays for DBM on cabbages.  You can check my publication

1) https://www.researchgate.net/publication/232322285_Resistance_of_diamondback_moth_to_insecticides_in_selected_cabbage_farms_in_southern_Ghana

2) https://www.researchgate.net/publication/267097603_Insecticide_Use_Pattern_and_Residue_Levels_in_Cabbage_Brassica_Oleracea_Var_capitata_L_within_Selected_Farms_in_Southern_Ghana

The second deals with residue levels of Deltamethrin among other pesticides

Article Resistance of diamondback moth to insecticides in selected c...

Article Insecticide Use Pattern and Residue Levels in Cabbage (Brass...

Thanks,

José Zanuncio