Dacron is usually IGFA standard, which means 30# would be really close to 30#. 30# mono is usually over-testing that even when wet and at the knot. Spectra/Dyneema usually over-test their rating.
That means, If I'm running 20-30-40# monos, I run my dacron at least 10# heavier than the top shot rating.
Dacron is about twice as thick as spectra and half the price.
Any reel having a graphite or wood arbour I back with at least 75yd dacron or more, depending on my top shot material. When I tie my dacron to the arbour, if it doesn't have that little stud, I lay a piece of adhesive tape over the first wind, (not layer, the tape should stick to the arbour.) This just keeps the line from the risk of spinning on the arbour.
My join knots are Albright for mono-dacron and doubled-line Uni-Uni for dacron-spectra. Some splice to the dacron I don't.
Cheers!