The amount depends on what bike and whether you change filter.
If it reads right in the window according to your manual then its fine.
Let it run for 30 secs, then settle and read when cold.
What ever, feels right and I just put on a Michelin crosscompetition (M12 XC), (who the hell came up with that spelling) it's been raining for 2 days and grassy *** hell,,, BBBBAAAAAAA, BBBBAAAAAAAAAAAAAAA, BBBBBBAAAAAAAAAAAAAAAAAAAAAAAHH.... I feel sexy.