Calculate capacitors for supply voltage to half bridge

those caps are there to stabilize the supply voltage at the transistor drains. depending on circuit board trace lengths, the values of the capacitors in the power supply, the load impedance of the half-bridge, the switching frequency, etc... the suggestions for what values to use are probably good choices. it's like choosing a filter capacitor for a power supply, there's a lot of "fudge factor", meaning a wide range of values that will work well. somewhere, i'm sure, there is probably a formula somebody has worked out for the optimum choice of what value capacitors to use, but the choice of values isn't critical.