This is already an existing action, I don't think it would be very useful to rewrite the code... From my experience scoring, movies most of the time it's a target tempo that you have and you play around with time signatures and hit points to get as close from this target tempo as possible.

EDIT: Here is the existing code, very incomplete and with terrible formatting but it's fairly simple so you shouldn't have to much difficulty figuring it out.

Code:

function main()
t_sec = end_time-start_time --gives the time selection length in seconds
a=0 -- 0 means only whole bars, 1 allows one shorter bar
t_tempo=121.00 --target tempo, user input
time_sig=4 --main time signature, user input (still have to include the denominator)
t_min=t_sec/60 --time in minutes
q = math.floor(t_tempo*t_min/time_sig)--number of whole bars we can fit, euclidean division.
r=t_tempo*t_min-q*time_sig --remainder of the eucliean division
inc_bar = math.floor(r + 0.5) --Round number, incomplete bar time sig
if a == 0 then
if r<(time_sig/2) then
tempo = q*time_sig/t_min
n_bars_norm = q
else
tempo = (q+1)*time_sig/t_min
n_bars_norm = q+1
end
else
tempo = (q*time_sig+inc_bar)/t_min
n_bars_norm = q
end
reaper.ClearConsole()
reaper.ShowConsoleMsg(" Result="..q.." ".."Rest="..r.." ".."incomplete time sig="..inc_bar.." tempo="..tempo)
end
--RUN
-- GET TIME SELECTION EDGES
start_time, end_time = reaper.GetSet_LoopTimeRange(false, false, 0, 0, false)
-- IF TIME SELECTION
if start_time ~= end_time then
main()
end