Is there a link where we can find the ideal routine to follow for 8months and older?
Nope, there's a big variation at this age, so there are sample routines of things that have worked for some babies but there' not a nice 3hr EASY style of ideal routine at 8mo. Here are the sample routines:
https://babywhispererforums.com/index.php?topic=164253.0Also, lets say their nap is at 10am but they wake-up at 10:45 am instead of 11:30am, should the 3h A time start as of 10:45 when they wake-up? Or should I leave them in their crib until 11:30am and start counting the 3 hours as of 11:30 but then they would be OT.
I'd try maybe 5mins of resettling and make a call as to whether you think its going to work. If yes, keep trying for another 5-10 mins, if not, get them up and count 3hr A time from when they get up. Don't leave them in bed for 45mins, its not going to be good for your sanity or theirs!
To try it out and see what would be the ideal activity time for them, should I start by a 3h A time for both and increase by 15 minutes every few days?
Yes, that's a reasonable plan. I'd do 3hr for 3-4 days, then bump to 3:15 for 3-4 days and so on til their naps get longer. If they get longer than shorten again, also time to increase the A time but know you're probably getting close.