It's not exactly rocket science, but you need to pull out the actual useful data.
'Combined rate of crossing' is 2000fpm (1000+1000), vertical difference is 5000ft (FL350-FL300), so two and a half minutes until both at FL325, and have either 'crossed', or let's hope not collided. If we're talking separation required, and assuming we need 1000ft, we need to add another 30 seconds (1000ft/2000fpm rate), so 3 minutes.
Do I do this on a daily basis? Not really. Maybe a couple of times a week I actually look at rates of climb/descent, think to myself for example '4000fpm closure, 6000ft to go, less than 2 minutes, that's fine', so I won't need to separate with headings.
I know it's easy to say but try not to overthink things.