Dacron is just polyester. Advertised breaking strength per strand is almost 50 pounds but in reality, it is often less than half that much. Perhaps your grandmother has some thicker polyester button hole thread? One string would probably take up a couple spools.
I'd go for the hemp or linen thread if you can find it.
-Alan
NO NO NO NO NO...
(AFAIK)
Dacron isn't "just polyester" that's like saying a microprocessor chip is just silicon.
Please please please don't mess with polyester thread or nylon or dental floss or any such rubbish which will stretch like the length of a fishermans catch.
Hemp, Linen or a decent bow string material.
Other natuaral materials are also available.
You are in danger of wasting precious bow making time messing with inappropriate string material.
If you want to spend your life researching NATURAL string materials then that's a fine and noble thing, but please step away from the artificial fibres (unless they are 'designed for purpose stringing material)
Hemp and Linen are
not poor second rate alternatives they are good materials. They were good enough to cut down the flower of French nobility at Agincourt on 150# plus draw weight longbows.
Del