Currently 1 US dollar is worth about 289 forints. That's pretty close to 300 forints, so a quick-and-dirty estimate can be obtained by dividing the forints by 3 and moving the decimal point two places to the left. Note that this will underestimate the dollar cost.
You see a sandwich for sale at a cost of 800 forints. Dividing by 3 gives you 267. Moving the decimal point two places to the left gives you $2.67. The actual cost is $2.77.
This is what I do when I have to deal with an ugly conversion rate like that: For items I expect to purchase frequently, like meals, I decide on a target dollar value I hope not to exceed and convert that to the local currency. I only have to do that one time for each planned purchase. For example, I might decide to keep my main meal of the day under $25, in which case I'd be thinking "7215 forints" as I looked at menus. That's much easier than converting the price of every interesting menu item into dollars to see whether it's less than my target figure--one calculation rather than many.