UnZudowned,
Until you are familiar with the machine there is this blue book normally just to the right of your head (when in the pilots seat) in a little box that should be placarded "Flight Manual".
Check out the Performance Section.
Do a few sample scenarios and derive a few "Rules of Thumb"
It beats guessing by a wider margin.
In some countries this is actually a requirement and may also be in yours. Check the details.