It's actually pretty pointless putting 'moderato' there, as a proper tempo is written. This gives exactly the speed of the piece, in beats per minute, each beat being shown as a crotchet.Back in the Classical days, composers would put the Italian words, which gave a rough to fairly good idea as to the pace of the piece.
Nowadays, it's more usual to put the actual tempo, in b.p.m and indicate what constitutes 'a beat'.
If 158bpm is 'moderato', then 81 bpm will be nearly half that speed, given that each uses a crotchet as the 'beat'.By the way, the number of beats in each bar will have no bearing on the speed the piece gets played.
On one metronome I have, 'moderato' is 108 to 120 bpm, so why that's marked as such on guitarpro, I don't know.