Welcome aboard John McLaughlin. I too need to have both knees replaced and wish I could. I've had Osteomyelitis since 1971 and no Orthopedic Surgeon will touch me with a 10 foot pole. How long have you been reloading? What loads have you tried in your 375 H&H? What size groups do you get? At your age, have you considered a scope for it? Due to the versatility of the 375, a number of us have two scopes with QD rings one low power i.e. 1.5 -4 x for closer work and something in 3-9 x or 4-10 x for use at longer ranges.
John here is a "paper" that might interest you. I use this and QuickLoad (computer software) to simulate performance of loads I'm trying to work up. To date they have been very accurate.
See the attached link http://www.the-long-family.com/OBT_paper.htm
Chris prepared a table of accuracy nodes for barrel lengths from 16 to 36 inches. If you were already aware of this please accept my apology.