Cards thickness is always measured in 'pt' or points which is a term used for the thickness of the card stock the card is made from.
This means there is a practical method to work it out!
Get some standard thickness base cards (20pt) and stack them up until they are the same thickness as the card you want a one touch for. Then, for every card you stacked up, add 20pt. So if it is 2 base cards thick, it is 40pt, 3 is 60pt etc
The. Round up to the nearest ultra pro magnetic size - eg a 40pt card would go in a 55pt magnetic, a 60pt card goes in a 75pt magnetic
The other method is to measure your card with a ruler in mm (millimetres).