Based on what I've read so far, I'm inclining towards reducing value of
bodies 25% a year;
flashes and accessories 20% a year;
lenses 10% a year.
I'll address "reality" - your accountant and tax man will tell you about the rules.
Your rates are "within the ballpark" but will vary not only with use but with your expectations and where you are positioned in the performance market.
If you want anything like top end performance and/or re working your way up you will probably want to replace bodies every 3 years at most and maybe even less. Consider the performance of 4 year old bodies now. The just-out Nikon D3 was high ISO king, 1ds Mk3 also just released. A 5D Mk1 was 2 years old. The fabulous D700 was 11 months away and not even rumoured. If you had one of those very fine machines then, would you have wanted to change it by now? Maybe not due to the very high performance even by most modern standards. D3 -> D3S or D4? - Maybe.
D3 -> D3x? But an A900 or A77 and ensure the ligt is bright :-).
1DS MKIII -> ... maybe.
BUT also just released then were the Canon 450D, Alpha A350, Pentax K20D and Nikon D300. Anyone who had bought one of those in late 2007 and who wanted reasonably current performance would have very itchy wallet fingers by now. ll have been noticeably improved on in a range of areas.
So - the lower spec cameras probably want replacing more often.
Also at issue is, what happens to the old body? Is it a backup or is it sold? Performance per $ with new DSLRs is always climbing, so that old bodies lose value substantially, but more so cheaper ones than high performance ones. You can still get silly high pries at auction for a D3. Not so an eg Alpha 350.
Similar things happen with lenses. VERY expensive lense are very expensive forever if treated well. A top lens will often increase in value because the new model that is not vast;y dearer is however much more expensive in absolute $ terms. A cheap lens will generally lose value. Good resource are the various enthusiast sites that rate lens performance and also have prices paid listed. For Alpha mount cameras www.dyxum.com is a superb resource. For Canon I know not, but such exist.
Flashes last long unless heavily used or abused, but I'd feel 10 years is too long. Guide numbers do not increase enough to be worthwhile on top flashes and once you have two axis tilt and rotate, high speed multiflash, remote trigger, chaining and a few more bells and whistles, odds are a new top version will not feel like a must have item. If you have a flash that works well for you and use it heavily, you can often pick up a used one with little use once they are "mature". Examples for me ar he Minolta 3600 HSD and 5600 HSD - the latter used to cost and arm and a leg but can now be had for only half an arm in good used condition.
Tripods die in many mysterious ways unless they have magic incantations engraved on them such as "Manfrotto". There are no doubt others which also last forever, but no name tripods or low cost name ones also don't last.
Yes, figures are an OK starting point.
Add the considerations re buy or keep as backup and
have I reached a performance level I am happy with yet?
I want a Nikon D800 (as yet not even named by Nikon) - hoped to be about D3s or D4 in dwarf's body. When I have that I will never NEED another camera until I wear it out. (Want yes,need no). A second one, yes. Some cheap others with fancy niche features? Maybe (eg I met a Surfing photographer a few days ago who had a 1Ds Mk_more but wanted an A77 Sony at 12 frames per second in a waterproof housing for 24 MP surf photos). At that stage the body replace cycle time extends to the length of the 5 year service contract.