"if i specify 8mm on my drawing will that be ok?"

I vote no. How about you? I'm joking.

You have given us ZERO information to help you. We have no idea what the part is, how it will be used, how it will be made, how critical this dimension is, how much it will cost, how significant failure would be, or any other inportant factors. Maybe its time you put your engineer cap on and take some responsibility for making a design decision, instead of just copying someone else's work.

For those who have spent hours scouring for the information to answer similar questions, myself included, I know that even saying it is plate makes no difference to a precise answer to that question. You need to know what the material specification allowed for the thickness tolerance on that plate; typically such specifications include many other factors, but often thickness tolerances depend on how large the plate is at the mill - the mill that rolled that plate is where the tolerance applies.

You also need to know what it's for - there is the chance the maker had a special mill run made just for this product or designed the product performance around that particular thickness, performance that will fail if it isn't that thickness.

There can be frustration to knowing engineering work that took a great deal of effort could be getting "reverse-engineered" without putting in a similar effort. Without doing engineering work, it's just copying.