Hey Anzer, first time poster here!
I've been through the same situation as you. I work a lot with very stringent budgets and small crews and have to justify every light I need, which is fine when you've done something similar before, but when I want to try something new or bigger I need to be sure!
My solution was an Excel spreadsheet. I would input a light's published photometric data into a column, say 1m = X fc/lux. Then using the inverse square law I would use that to extrapolate the foot candles at all different distances. I then had another column that converted that foot candle measurement to camera stops. As an example, an Aputure 300dii open face at 3m, gives about 120 fc. The spreadsheet calculation works that out as T83 (assuming 25fps, 1/50 shutter, and iso 800). That’s the starting starting point for me. So on a recce for example I would determine my light would likely need to be 10m away, and I'd know I'd want a key stop of T4. Using the table, I can see that the Aputure open face at 10m gives T27 - quite a bit under what I’d need.
Though to be honest, it's not really that elegant and quite time consuming, and the formula for converting footcandles to camera stops is complicated. If you're mathematically minded there's a few websites out there that can help. I'm not, so I had to ask a friend to help. It also doesn't take into account a lot of other factors (diffusion, existing ambient, spread etc). In it's favour, you do get a good idea of a unit's falloff over distance which is useful.
And despite having all this data, real world experience beats it all, as the others said, get to know a rental house, try out some lights, and makes notes about your fixtures on every shoot Hope that helps!
