is there something on the meter itself that tells you how much power your flash should have for a proper exposure?

ok thanks for that point.

here is a good picture: https://www.amazon.com/Sekonic-Flashmate-Digital-Incident-Reflected/dp/B000BTCJNG#

the meter shows you an fstop that produces the correct exposure with the metered light. you can change shutter time and iso. the fstop will change in relation to them.

so if you are in a room with a window. you can meter the light that comes in and it will show you the right fstop you would need to expose a model standing in this light. depending an what iso and shutter time you dialed in.

so in terms of flash. you meter the shooting flash and adapt the settings of the flash to meet your requirements.

coming back to your initial question. you could calculate and shoot without metering or trial and error. but there is no reason to do that other than beeing sure your lightsource is strong enough or not to strong for the thing you are planing to do.