I actually use 300 days to calculate, nowadays.
I have one mare that goes just under 300 days every time, and then some that go around 310 ish.....it is always a bit of a worry!!
But about the breeding when in foal...YES definitely, I had one complete line of mares, daughter, mother, grandmother and two close cousins, ALL of which cycled when in foal, and the three closely related mares took the stallion at least one month into their pregnancy, and one mare took the stallion three cycles into her pregnancy and drove him NUTS for two more months squirting and flirting but refusing him when he offered.
This was one of the main reasons I stopped "natural breeding" as the stallion finally lost it and beat the mare up quite badly...I was a bit naive at the time and had not realised what was going on.
The next year I had her Ultra sounded and so I know she was in foal, and then watched her, and was thus able to work out what was going on.
Very annoying, especially if natural breeding is your only option.
Nowadays I do both, so the stallions get to run with mares and foals, but rarely actually breed loose, and I have the advantage of knowing exactly when each mare was bred.