As u/elkster88 said in the other thread, part of learning how to use a feeler gauge is developing a feel for tight, loose and nominal. That feel when using the go/no go method can throw off the end result if you get it wrong.
It would also help to know what you are using for measuring equipment.
Your math is right but you likely have a error in your feel and maybe measuring equipment and a tolerance stack adding up in your measurements. Also never trust the number printed on shims. It's not often the marking is wrong but it happens and once you get into bikes that have had multiple adjustments, some times by many people you may run into shims that have been dressed(ground, sanded) down to a thinner size to achieve the desired goal.
When you are making your initial measurements you need to also need to be mindful of the oil film on the valve train components. That oil film can add to your tolerance stack by changing the feel or gap. For instance on valve operated by rocker arms a seemingly tight valve can "loosen" up by pushing on the cam follower side of the rocker arm thereby squeezing out the oil between them. There is also a oil film on the buckets and shims that can slightly alter your measurements if you put the shim/bucket back in totally dry. If the cam is being pulled to access the shims, putting it back dry is a no no and you have to make sure it's properly torqued and seated before your remeasure.