I tried a number of different size restrictors in return line starting with the recommended size but that back pressure pushed the fuel pressure too high, so I started increasing size incrementally until it was within limits. This ended up being quite a large bypass hole. Recommended factory return line restriction is 0.35mm
mine is 1.5mm in order to keep fuel pressure in limits which is over four times the normal. On the up side, there is less chance of getting a blockage 🤔