time prediction?

[McShieldsy]

I have heard that for a good 400m runner, your 400m time should be your 200m time - doubled + 3.5. 24.3 x 2 + 3.5 = 52.1 which is exactly correct for me.

But i was wondering what the rule of thumb is for the 800?
What would you add to your 400m time doubled?

[HBAC]

its hard to say...your 400 time means you have enough speed to run around 2:00. Though this all depends on your endurance. You would certainly run around 2:05