Would the 30-minute minimum check-in time at St P be part of the 35-mins minimum connection time at London International (CIV)? Or am I misunderstanding?
If it is, then it'd be an arrival at Marylebone 30-40 mins before that (depending on time of day), ie max 75 mins before St P train departure.
[As per other threads: where the recommended "before departure" arrival at St P is massively longer than the 30-minute gate cut-off, the experience of many people suggests that such early arrivals just clog up the system and make matters worse. There seems to be a consensus that 30 mins (absolute max) prior to the 30-min gate cut-off is more than enough. However, I appreciate that the question here is about technical connection times to maintain ticket validity, not about when is, in practice, a good time to turn up at St P.]