When you measure your OG, is it appropriate to measure it with the trub (and hot-break) in the fermenter. This would give you a higher OG than if you racked off the trub and then topped off to 5 gallons, right?
Trub doesn’t matter, but topping off does. Since trub isn’t dissolved, your OG is the same with or without it. But adding that top-off water is sure going to make a difference. But the difference is linear. So if you have 18 litres, and top off with 1 litre, you simply adjust your OG number by multiplying by 19/18 or 18/19 (depending upon which way you want to convert).
After addition of top-up water OG will be lower than before.