I’m a fairly new whistle player, and I just ran into my first F natural. Are half-holes supposed to be played with the pad of the finger, or with the finger straight like recorder players play them?
For F nat I ususally use the tip as I find it quicker and more accurate. I’m not sure I would call the finger straight though more like 45 degrees.
At speed I used the finger-tip. Don’t worry about half-holing. It’s rare. YOu’ll pick it up later.
At slower speeds you can get a nice sliding effect, especially on C, by using the slanted 45 degree finger technique. Works nice on Planxy Fanny Power.
I use finger tip for most, except C nat, where I roll the whistle slightly, or flatten out my left index finger.
I tend to use the pads of my fingers…close to the tips. On my A and lowD I use a piper’s grip and I have to halfhole my Fnat by angling my finger up…I’m just glad I don’t have to do it very often(the stretch on my Overton is tough)
I just don’t play tunes that need F naturals.
What’s the fun in that? There’s some really nifty tunes that have Fnats. “Colleraine” is one I can remember off the top of my head. Alot of tunes with Fnats seem to be in minor keys and I really dig darker sounds. Try playing tunes with D#s! THAT is a pain!
Keep practicing as D# is actually very easy on high whistles.
I find playing with half the fingers helps me! 
I roll my finger over just a bit.
With pratice, it’s not so bad.
Some whistles half-hole better than others…if you’re not having luck with one kind after serious practice, you might try another.
–James