dear OP
imagine you are taking off and your landing gear fails to retract
do you: stop, and look at data
or
DO YOU FLY THE PLANE?
It would be really rare, though not impossible, to have landing gear fail to retract and also lose an engine at the same time, so you will probably get away with it.
Now, it may interest you to know there are 4 segments of climb and it is not just about losing an engine.
IN REAL LIFE big airlines have engineering departments that make all sorts of calculations for each airport and each runway so we really don't worry about it too much.
But if we had a takeoff from a non routine airport, we would have to examine things.
Pilots sometime have to just "wing it" or play it by ear.
Flying is an art as well as a science!