Locker rooms usually exhaust what they needed to, 1150 in your case. They heated/supplied as per load, 800 in your case. The balance of make up air would be brought from the gym thru a doorway with no door, just a line of sight partition. The 350 cfm difference is brought thru the other gym systems( in addition to the required rate of .3 cfm per sq/ft?) and pre heated avoiding complications, and well diluted. Most gyms are short on fresh air during hi activity periods, so the method I described has 2 benefits.

You'd have to confirm with your local building official. The may want all make up brought directly to the locker room.

 

This locker room is next to office space and a eating lounge. So I believe I do have to supply the make up air. It's for workers at a plant. But if I supply 1125 cfm and exhaust 1250 and only need 800 for the heating requirements, my total supply air (for ventilation and heating) would be 1125 cfm? Or do I need to supply 1125 cfm for ventilation and add another 800 for heating to bring the total load to 1925 cfm?

 

Your total in should be less than your total out of 1250.

The attached section 403.7 applies to my state. I can use a variety of make up sources to match my exhaust total. The tables determine the air rate changes for various occupancies. Again, you'll have more luck avoiding discomfort by transferring tempered air than to bring in untempered air directly from outside to the locker room.

If you have a return in the room for some strange reason, that's more "out" volume. the 800 supply figure won't add to the supply total and cancel. you'd have to bring that volume in additionally, from somewhere.