I have some experience with front-end, also Angular, and NodeJS. So I’ll try to answer each of your questions clearly and unambiguously.
How many front-end developers are needed? - 1, maybe 2 (depends on scope of work and time limit). This will be enough because you are going to use cross-platform development framework such as NativeScript (if I understand ...
I'm just beginning, and despite my search, I cannot find any simple formula that would return a number between 0 and 100%
Let's look at the meaning of the numbers first, because I would instinctively not expect a 0-100% range to be the most valuable metric here.
What you're interested in is accuracy, i.e. how far you were off target. The ...
For accuracy the way you seem to think of it (as a number from 0 to 100), the simplest is probably
100 * MIN(actual, estimated) / MAX(actual, estimated)
where MIN(A,B) returns the smallest of A or B; and MAX(A,B) returns the largest of A or B. This is equivalent to Sarov's answer.
The problem with this formula is that it isn't linear. For example, ...