Rimfire ammo is night and day from centerfire ammo.
A chrono is a must have tool with centerfire ammo, because you can control everything, but on rimfire, it's just a nice to have as you can control nothing.
Not even the speed value that comes in the box can, majority of the times, be reproduced.
That said, and if you have one, and desired to take some tie using it, I think the best way is to choose the slowest ammo lot between what's available, considering it shots well. You will also find values that will puzzle you, meaning, spread values but ammo still grouping.
I'm sorry to say, but a chrono is of very little value on rimfire ammo.