What determines pressure is bullet seating depth. You math is on track. Determine the bullet seating depth of the load listed in the manual. If you use a bullet with a different length (but the same weight) adjust your OAL to provide the same bullet seating depth for equivalent pressure results.
I have an older Lyman manual that did list bullet seating depth for each load. But it is easy enough to determine and adjust. For bullets you don't have the length of, this link
bullet length should prove useful.
It gets discussed all the time. I have seen at least 5 threads in the past week on seating depth here.