Well, say you want to buy something from someone for 1g (1000s).
If you were buying and they wanted you to pay 1g before tax then they would receive 700s (1g - 30% or 300s)
If you were buying and they wanted you to pay 1g after tax then they would receive 1g, and you would have to pay 1.43g (1430s or 1430000 bronze/normal Mushcoins) (approximately).
Using Jonno’s formula, you can see why this happens, since 10/7 (or 100/70) is equal to approximately 1.43.
Basically, to calculate how much you have to pay if the person you’re buying from wants a certain amount after tax, multiply what they’re asking for by 1.43x and you’ll end up with roughly that amount.
This also works in reverse, with the rolls swapped. Say you’re selling something for 1g. You’d receive 700s if they paid exactly 1g, and approximately 1g if they paid 1.4g