the formula I kept finding [and used] was ...
W x L x H x 60 x 1.33
so ... 1.2 x 1.2 x 1.5 [tent dimensions]
x 60 [air changes per hour]
x 1.33 [allowing 33% for filter 'drag' and ventilation 'headroom']
= 115 m3/hour
Your filter will need to be at least equal to this - preferably >25% more so you're not running it at capacity.