Best and most trusted 1×9 chain length calculator I’ve used is –
Shift the bike into 1st.
Remove shock spring/air from shock.
Compress the suspension.
Fit the chain, add or remove links until the mech is almost at full stretch.
If its 1×9 on a hardtail then
Shift the bike into 1st.
Fit the chain, add or remove links until the mech is almost at full stretch.
job done.
been using this method for 10 years
you will find if you “guess” the chian length the chain maybe too short this will have a massive effect on suspension action. the chain may stop the bike getting full travel. leading to snapped chains or at worse ripped off mech hangers from frames. too slack and the chain will flap around all over the place