New hornbeam hedge pruning questions - how much, how often

Hi all,

Last November we had around 100 metres of 60-90cm Hornbeam bare root whips planted at 5-7 per metre.

We're ultimately wanting them to turn into one tall hedge that can't be seen through for the most part. To encourage this, I plan on pruning the outward growth while leaving the leader alone and wanted to clarify a few things.

No initial pruning was done when the whips were planted (which may have been a mistake). They seem to have settled well and started putting out some healthy leaves and growth - they had rootgrow applied to the roots and bone meal mixed into the compost and regularly have a liquid seaweed tonic applied to the leaves.

In my calendar I've had the start of June and September marked as the time for pruning, with the latter understood to be to encourage new growth and retaining the leaves over winter.

I think where I may be misreading certain advice is formative vs maintenance pruning and what to do now in the formative years compared to when they're established.

I've read that pruning around 1/3 of outward growth is recommended to encourage bushy growth (which seems daunting, but want to do what's best for the end goal). I've seen mixed results on whether this should be done when the plants are dormant or can be done in the June prune, or both.

Should I be doing a combination of these during the formative years? Pruning in June, September and when dormant?

I'd appreciate any input on how much and how often you'd suggest pruning.

e.g.

June - is 1/3 too excessive if also doing when dormant? Just back a few buds instead?
September - Just to nearest bud, or more?
Dormant - 1/3?

Thanks in advance.