1RM on Wide grip curls - 21's

[MidgetDogg]

I'm redoing my 1RM tests today in preparation for starting another round of STS. I didn't do a test on the wide grip barbell curls before, and it's a 21's set. So, if I chose a weight of 30 for my test and was able to do all reps (7, 7 and 7), do I say I did 7 reps or 21? If I put 20 in, it calculates my 1RM as 64. There's no way, so I know that's wrong. But I'm wondering if I'm calculating too low if I use 7 reps.

 

[SNM Videos]

1RM testing and 21's

You really can't use 1RM testing to calculate the proper weight for a set that uses a rep pattern like "21's".