Try this one.  Free download with no sign up required.

https://www.mediafire.com/file/5mcemcyutrm/The+Air+Gun+From+Trigger+To+Muzzle_s.pdf

https://www.scribd.com/doc/124078012/CARDEW-The-Airgun-From-Trigger-To-Target

I've never had a rifle that's so easy and so pleasurable to shoot.   JSB Jumbo 15.89gn Diabolos and bags over the tray of the ute.   Not that I've had a whole lot of them anyway.  1 Milbro, 1 BSA and one Hatsan, none of which had tuning kits, not that they were a thing for the Milbro or the Meteor 55 years ago.

I'd been sighting in the .25 Gamo Magnum earlier and though that's accurate enough, it's a bitch by comparison.  So uncivilised and no 1 hole groups.  No tuning kit either, other than replaced trigger screws to get the let off down to around 2lb.

The chosen rifle was a Sig Sauer ASP20 in .177.

Design power was 20 ft/lb which was excessive for target shooting.

Calibre

.177 calibre has the better pellet selection available to experiment with.

.177 at a lower cost allows for economical practice.

.177 unit was tested against an identical .22 and proved to be more accurate indoors.

Stage 1

Pellet JSB Exact 8.44 grains .

Piston Seal replaced with a 30mm Custom Seals HW80 short stroke kit.

Internals checked, cleaned and reassembled with dry graphite lubrication.

Bipod fitted, trialled and removed.

Fitted cheek riser.

Outcomes

sub 12 ft/lb 

result 80/100 target scores

Stage 2

Pellet Air Arms Diablo Field heavy 10.3 grains.

Trigger adjustments to shorten second stage.

Fitted recoil absorbing shoulder pad.

Outcomes

Rifle is less demanding and easily able to complete 2 rounds of 20 pellets.

Result 90/100 target scores.

Stage 3

In progress, 7 months to go.

For me rheres only one anzwer for those platforms Maccari full power, thats what i had installed in my diana54, never regretted it.

you get the best useable power as smooth as it can be 

But another thing that can be added is a piston extension that "short strokes" the firing cycle. This will make your shot cycle snappier and reduce power a touch due to less air volume being pushed. I quite liked the feel, and reduced recoil. thay are approx 10 mm. But consult someone like Nick at Tinbum tuning, as he will know what you need to get the result you're after.