The thing you’re doing here is establishing a baseline:
You need to look at the session afterwards and count the number of mins above 90% MaxHR. Too few and you need to bump it up or go longer per rep. Too much (like you can’t complete workout or you reached actual MaxHR) and you need to dial down. In ERG mode you’ll have to be quick on the trigger.
Don’t flog yourself. It’s about time in zone at VO2max. See if you can get 12-15 mins total and progress from there.
You can play around (later) with 90% MAP or whatever percentage as long as you adjust duration. The trick is getting those TiZ minutes