My benchmark for the 200 free is 16x50 on 20 seconds rest. I try to hold an even pace, all the way through. If I can do that, I figure I can hold that pace for the last three 50s of a 200. Then I figure the first 50 will ~3 seconds faster. For example, if I can go 16x50 at < :33.00 on a :53 interval, I have confidence I can go a <2:09 200. (I use a SportCount finger stopwatch to get accurate touch times in the validation set.). If I don't have my wrist watch to program an exact :53 second interval, I'll go :55 off the pace clock, but I'll also look to get to 18 reps to confirm confidence that the pace will translate to racing. I have found a very high correlation from this set to race performance.
For 500y free, I've found the sustainable pace of 16x75 on 20 seconds rest to be pretty predictive of an overall average race pace.
Can only speculate on 400 yards since I've never raced that, but I'd venture an educated guess that if you could do 16 x 75 @ <:42.00 on a 1:00 interval, you could do a 400 in 3:40.
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