Good luck on this job. Would ease up a tad on the fertilizer now.
In the end, the result is going to depend more upon the health of the trees involved, rather than the timing up to midsummer. One wants to fertilize JPB in development strongly in spring and ease off once candles are cut, to decrease eventual needle size.
It is a normal practice for robust and healthy younger JPB to cut back into last years green needles each year to develop backbudding. Also have done this to older JBP and JRP, but in late June.
We have done this process for three years straight as late as July 20th in our area with outstanding results. It’s also in Eric Schrader, Developing JPB for Shohin course, which I strongly recommend taking a look at as this project is similar in scope to his work.
Cheers
DSD sends