USRPT training - how should I identify goal time?

I am going to add some USRPT training later because I love the idea of fast feedback. I tried a set last October, with target time 2'5" / 100 m (5 seconds faster than the pace on a recent 1500 m T/T) (although not strictly in adherence because I started every 2'20" instead of 2'25" for 20 seconds rest) and within a week I pushed my first failure from 8 to completing all 30, requiring me to "level up". I tried reducing the target time by 5 seconds / 100 m but it became too hard such that I couldn't get through the initial 5. Afterwards the pool I used closed so I didn't continue this training, but I am looking to resume it when the pool reopens again next week. I will take a 1500 m T/T the first time I get back to the pool. I consider that a pace is too easy if I can complete all 30 reps without a failure, and too hard if I can't get through the initial 5. How should I identify my initial goal time? Is directly using the 1500 m T/T pace good enough, or should I reduce a few seconds? And if I need to level up, how many seconds should I take away for each 100 m? I am currently targeting my 1500 m speed in these few months, however eventually I will need to race for 5 km a few months later, and 15 km next year, and will need to modify the workout for that? Does anyone have any hints or should I return to traditional training by then? I want to build up my speed first, then endurance second, because I can swim for 5 - 7 km continuously without problem now, but at a very slow speed (around 2 hours for 5 km).