Adjusting the B screw
Have a look at
this thread
I don't believe Shimano's 90 degree to the ground rule works all the time. If you look at Shimano's tech docs, they admit there are exceptions (e.g. when the smallest rear sprockets are beyond certain size), and I would have thought it also depends on how far one pushes the rear mech beyond official spec., or how far one needs to push an existing chain to fit.
I always use a calculator, easy enough to use and the theory behind it is far more robust, imho.
To answer the OP's question, the size of the small sprockets front and back have nothing to do with the minimum acceptable length of the chain (it being determined by the chain's ability to wrap around both large ones while leaving some slack for the rear mech). Hence in his case IF the existing chain is at the limit he will need to add, not remove, 2 links. On the other hand, one can use a chain longer than optimum if one wants as long as the rear mech has adequate wrap range (meaning long enough cage) to take up the slack. One might want to do the latter, e.g., if one might get a bigger large sprocket front or back later.
A potential issue the OP might encounter, depending on the spec of his existing rear mech, is exceeding its wrap range (since it is going from 21T to 30T). Adjusting the B screw can help a mech clearing the largest rear sprocket (especially if it is beyond the mech's spec), but exceeding the wrap range has implications on cross-chaining, reading the relevant section
here may help.
