Laura, Depends on the seeds many plant and flower seeds are very fine where as Vegetables are bulkier. I watched my Dad as a lad steaming garden soil or heating it in old biscuit tins on his greenhouse wood burner for the fine seeds sown in old fruit boxes no plastic back then either. He lost very little after he sifted the sterilised soil and mixed it with washed sand, we lived next to a sand pit and he washed it himself, no GC or bags of grit and sand, "gosh" they were hard times.
I buy John Innes number two compost which is soil based then mix my own to suit what I am doing. One third compost one third fine grit one third washed sand for seeds, they contain their own nutrient and do not need any feed until they shoot, they will seed in sand I have done it many times.
Prick out into half compost and half made up of washed sand and fine grit, that is enough nutrient and the sand and grit gives drainage.
Pot on to three quarters compost a quarter sand and grit it works for me and as an engineer if it works then leave it. Many seeds are expensive why risk them cutting back on compost.