This is the situation in which dive computers are most helpful.
Until then...

You must treat the entire dive as if it were at the greatest depth.
Yes, this is probably overly conservative, but it is the only thing
that the tables support.

OR...

There are multi-level algorithms available for some dive tables.
e.g.  PADI's "The Wheel".  They allow you to calculate the dive
as sevearl different depths.  e.g.  10 minutes at 35M; 15 at 25;
and 15 @ 15.  (All depths viewed as "not deeper than".)  The practicality
of these systems is questionable.  It can be difficult to execute your
dive following a rather complex plan.

CAUTION: Don't improvise your own multi-level algorithm!  All dive
tables include various assumptions that must be taken into account
when they are used for multi-level diving.  These assumptions and
how to account for them are seldom obvious.

SUGGESTION: Borrow a computer with a planning function.  Us it to work
out a safe profile for the situation you plan to dive.  (See the example
I gave above -- although that is off-the-top-of-my-head and may not
actually be safe!)  Then follow that profile.  (Work out a profile
for a second dive too, if you wish.)

As you already know, the longer term solution is to buy a dive computer.

(snip)

Easy answer:
Goto:

http://www.suunto.com/pls/suunto/suunto2.pubmainpage.frameset

and download Suunto Dive Manager software. (for free!)
Then run a few simulations of the kind of dive you're worried about
and you get all the answers.

Yes, that would be a good idea, but running your dive profiles dry is
a very, very safe way to find out how you actually get loaded with
nitrogen and learn how the computer would work for you.

Buying the computer is oly one thenth of the solution.
Understanding what it does is definitely the more important part.

HES van Schoonhoven
Life is a learning experience

Pelaius left this mess on Tue, 2 Mar 2004 17:25:21 +0000 (UTC) for The Way to
clean up:

Your bottom time ends when your final ascent begins, ergo, your bottom time is
whatever time you spend at 40m. You are correct that your entire dive then
becomes your ascent, which should be done at the appropriate rate you were
taught, except for the times when you stop at 10m. In those cases, your bottom
time is the time up to your ascent from 10m.

Unfortunately, in tables, you wouldn't get credited for the ascent from 40m to
10m. You'd need to calculate your dive one of two ways. Conservatively, you'd
treat the dive as one dive to 40m.

But, you *could* treat the dive as two dives: one to 40m for however long you
remained down there, zero surface interval, and then another dive to 10m for as
long as you remained there. This would give you a little more dive time without
crediting any surface interval, and would mimic the results you would get on the
PADI wheel. In planning the dive in this way, however, it is absolutely
essential you keep well within the NDL for the "repetitive" dive. You have much
less margin for error, even tho the margins are generous to begin with.

And of course, any dive below 30 feet should be ended with at least one safety
stop at fifteen feet.

Tao te Carl
"It takes a village to have an idiot." - Carl (c) 2003

(Kudos to Cap'n Jim Wyatt for this link) BEFORE you ask a dumb-a.s question
here...http://www.speakeasy.org/~neilco/bart.gif