Assumptions
The mitten shall have even numbers of stitches.
There shall be a number of extra st between the hand and thumb. These are assumed to be the number of hand st divided by 12 and adjusted to nearest even number.
Ex. If the mitten hand has 46 st, the extra st will be 46/12=3.8 => 4
The thumb circumference is assumed to be double the hand circumference devided by 5. Number of stitches is adjusted to nearest even number.
EX: If the mitten hand has 46 st, the thumb will have 2*46/5 = 17,6 Increased to nearest even number = 18
The thumb is assumed to be placed in a correct position when two increases are made every second round from the cuff until the number of thumb stitches are added Starting and ending with a round without increases.
Four decreases every round until few st remains is assumed to give a proper ending of the hand part.