The first shot is the most important shot you are going to fire.
Many full moons ago, I'd foul my barrel, then load her afresh for the hunt. That meant I had to discharge the rifle and clean it, every day. Then, I merely worked up my most accurate load (shooting fouled) and then tested to see where that first, clean barrel shot landed. It is usually within a couple inches of the 'fouled' shots, and usually just a bit low, so no allowance was needed on rib-cage shots - allowance only on head shots, which I stopped using a few decades ago as in my opinion, head shots are the use of poor judgment. I found it easy to remember the first shot would be a couple inches low - or wherever it was left right, high - but usually low. A couple inches is easy to hold for, on a deer's ribs or the 30" kill zone of a moose or large elk.