This is a spreadsheet tool I threw together in order to make calculations regarding the mass ratio between payload, spacecraft dry mass, and propellant when developing add-ons. You type in the masses of verious spacecraft components and payloads and propellant, type in your engine parameters, and it tells you how much delta-V you have based on Tsiolkovsky's Rocket Equation. The second worksheet allows you to request a given delta-V and then tells you how much propellant you need based on your engine parameters (Isp and thrust).
The inputs are in English units, but if you unprotect the worksheet you can fiddle with the cell formulae and change that. I built this tool in about 20 minutes, so it's very simple.
It's not fancy and was never meant for distribution, but some folks asked for it on Orbiter-Forum, so here it is. Use at your own risk, etc. Hope somebody finds it useful.