This sounds like a blow to the punchbag - as in boxing, but that does not fit the arts and crafts context.

A far more usual expression is
et slag på tasken
--- maybe influenced by katten i sækken?? (= a pig in a poke)
We have just celebrated Fastelavn, when the game is to
slå katten af tønden...

Anyway, if what was intended was ´et slag på tasken´, then
it means a rough guess, possibly knowlegeable, but not based on exact calculation.

Just a few thoughts out of the top of my head :-)