I am trying to figure out the mathematical solution of your lineage to be 45%
Assuming that brothers and sisters don't mix (marry), etc etc.
Everyone has 4 grand-parents, each of whom have 4 grandparents themselves.
16 Great Great grandparents.
So if 8 of them come from Belgium, except that one of the 8 was born elsewhere, French/English/German etc etc.
8-1 = 7
7/16 = (Approximately) 45%
(0.4375 if you are mathematically pedantic).
Maybe a Genealogy program or web thing, produced the 45% figure for them.