looks like between 8 and 12 minutes at ei 100 is good middle grey is in the middle
looks like 10 minutes should be about right. try that.
You also may want to try 1:1 diluition for 10 minutes--it sounds counterintuitive, but some developers need to be dliuted with water to get more power.
don't worry about them other people's times...this is why people give up. everyone elses times always 6 minutes. the 6 minutes is for people adding hypo probably. stay away from that for now. You are on the RIGHT track experimenting with times higher than what other people say. Anything like over 10 minutes, watch for fog developing and overall dmax being lost. just increase the times till your happy or till you start to see ill effects of dmax reduction from developing fog due to too long in the developer.
more diluted may give you a more pleasing contrast too--that may be playing tricks with your eyes as well....
Also--you CAN try to recover some speed by using a ferricyanide (dilute) bleach step after processing if your results are too dark. You can save underexposed film that way--AGAIN at the expense of dmax.
you may want to take pictures of a face or hands or something when you test for a better reference.
actually--on second looking...the 8 minutes looks good per the greyscale