With the help of Wolfgang (thanks) the solution is (tested)
\setupformulas[prefix=yes,prefixsegments=chapter:section,way=bysection]
To all interested
I found a simpler solution, not knowing how prefixsegments really work, but mainly understand
from this example, is
\setupformulas[prefixsegments=chapter:section]
instead of using
\setupformulas[prefixsegments=1:100]
Both setups gives the same numbering scheme of the formula numbers.
Both setups also don't reset the formula number after a new section, it just
keeps numbering the formulas
I know I saw a code snippet which does some sort of reset, I think prefixreset or reset, but
I cannot find it anymore.