Question about HIIT and calculating target heart rate


Please can any of you offer any words of wisdom over the best way of calculating a target heart rate, it's actually driving me slight nuts now.

I've looked at various formulas, the old 220 minus age one (conducted in the 70s, mainly on men as far as I know), the variations of the more recent studies conducted on women (200 - 67% of age and 206 - 88% of age). And these all give similar results, the final 2 obviously lower to account for the newer findings showing that females had a lower max heart rate than males.

So this is fine. But then there's also the one which factors your resting heart rate into it. Supposedly this is meant to cater more to the individual, however when I use this formula, my supposed target heart rate %ages rocket sky high.

Examples being, 60% using the first 3 methods mentioned come out at between 101 -106 bpm, using the last RHR method, it comes out at 138 bpm. 80% comes in at 135-141 bpm with the first 3, then shoots up to 158bpm using the RHR method.

This is a huge difference, 60% of the RHR is pretty much equivalent to 80% using the other 3 methods. Also, I truly think I'd give myself a heart attack if I tried pushing myself to 158 bpm. I'm not overweight, I've been working out 5 days a week consistently for 8 months now I think? And my HIIT workouts give a bpm of around 135 normally (however I'm not laid dying on the floor after them, but I do feel I couldn't carry on for much longer at all after the 15 mins or so).

So! Have any of you found a method that is reliable? I really don't understand the huge leap with the RHR method... I know that the only real way of knowing your own is going for a personal physical stress test, but I'm just looking for a reliable-ish way to roughly calculate what I should be expecting for the %age I'd like to hit.

Thanks for any help and sorry for the long post!