how do you compute for the fetal age using the second typical menstrual interval (LNMP)?

I answered this in your concluding question. So I only just copied and pasted.
Yes. You count from the sunshine your last time of year started. This isn't very accurate, though. They also carry your due date from this. Start day of later period- 3 months and +7 days. Example: last time started: August 10, - 3 months= May, +7 days= 10; due date is May 10th.
Count forward, from the first day of your second period. Add 10 days to that, consequently add 40 weeks to acquire your "due date." gestation = due daye minus toady.