Most carburetor manufactures make their jets so that there is approximately a 3% to 4 % change in fuel flow if you move one incremant in main jet or pilot jet size.  Moving the needle one clip will usually affect fuel flow approximately 3% in the throttle position that the needle is the primary control circuit.

Mikuni main jet size numbers represent a flow rate at a particular test specification. (suction on the jet).

Small Mikuni jets have their increments in increments of 5.

Larger Mikuni jets have their increments in incremants of 10

Many years ago I could buy main jets increments of 10 all the way up to 720 and increments of 5 below about 260.  Now they only offer increments of 20 on main jets of 500 and up and increments of 5 below 200.

Going from a 190  to a 195 main jet is a 2.6 % change in fuel flow.

Going from a 200 to a 210 main jet is a 4.8 % change in fuel flow.

Going from a 300 to a 310 main jet is a 3.2% change in fuel flow.

Going from a 400 to a 420 main jet is a 4.8% change in fuel flow.

Going from a 500 to a 520 main jet is a 3.8% change in fule flow.

Going from a 600 to a 620 main jet is a 3.2% change in fule flow.

Going from a 700 to a 720 main jet is a 2.8% change in fule flow.

Lookin at the above information, I think that the trend is easily observed why some jets are offered in steps of 20.

When I am fine tuning I do not like to make more than a 4% change in fuel flow.