Another point to bear in mind when discussing this subject...
First solo in a heli is generally a 5 minute hovering exercise whereas first solo in a plank is a full circuit. I would suggest a better statistic is first solo circuit in a heli as this is a better measure of wider ability.
On the subject of heli to plank conversion. Have a look at NPPL as you only need to do sufficient hours to pass the test. Could only be a handfull. I know it is more restrictive but it is a start.