It really depends on the components of your cocktail. Factor in how much lysis buffer you're going to use with respect to the mass of your sample as well.

E.g. for a 5 mg sample, let's say you want to use 1 mL of buffer (to keep the calculation simple). To make things simpler still, let's pretend like you only had a single protease inhibitor, pepstatin (MW = 685 g/mol = 685 mg/mmol; density 1.12g/mL), whose working concentration is 100mmol/L. From a random study, let's say that 1mmol/L of pepstatin in the buffer worked out nicely, so you decide to try that on for size yourself.

In a 1 mL buffer volume, we can calculate how much pepstatin we should add.

1 mL buffer / x mmol pepstatin = 1000 mL buffer / 1 mmol pepstatin; x = 0.001 mmol of pepstatin for your buffer!

We have a working concentration of 100mmol/L.

0.001 mmol pepstatin needed / x mL sol'n = 100 mmol / 1000 mL

x=0.01 mL= 10 uL, using your favorite 0.1-10uL pipette!

You could also factor in the displaced volume, but it's negligible, really (after all, we're talking about a total buffer volume of 1 mL here, which is 1000 uL). Do this for every component in your cocktail. As I recall, I think the total for one of mine came out to 25-50 uL, but it has been a long time.

I hope that helps! Please correct me on my math if you find any mistakes.