The time to fly a route = ([Route Distance] * [Number Frequencies] * 2 / [Aircraft Speed]) + ([Turn Time]* 2)
Would I put the frequency I want to fly there (So 1, going on from what I learned from my other thread)?
And this would tell me how long this new route will take? How would I compare that to what's left on my plane?
Yes, that is called Block Time (time to travel one-way).
Here is another version of the same formula...

After you calculate the Block Time, look for the aircraft you want to fly that route in your
View Aircraft page. If the Block Time is less than what is left in the "Hours" column, then the plane can fly that route.