As others have said, keep it simple. First I would spend enough range time with this set up to know where a shot would hit at different ranges. And to make sure I could hit my mark.It sounds like you have that covered.
Some quick ballistic calculations show that a sierra SSt 300 gr from 0-100 yds you should be within 1.5 inches of your visual bullseye. Placing that shot in the boiler room should result in a dead and recovered deer.
I don't use a slug gun, but I do use a muzzle loader. I use a 250 gr .50 cal (.45 projectile) and my aim is always the heart lung area. If I hit my mark, they very seldom run more than 20-30 yds. More than half of them have dropped within 10 yds of where they were hit. I have only lost one deer with muzzle loader. A nice buck that was the first deer I shot with muzzle loader. I had no scope on that rifle....I'm pretty sure the fault was my shot placement being high.
The only other thing I can think of is that 25 yds is very close unless you have a heck of a lot of cover. Deer are always busting me.