Ducting Systems and Air Ventilation - 18' dryer duct, 2 90s

Hi all. This is my first time posting to this particular forum. Roughing in venting for a future electric dryer. I've done my due diligence, and I know that standard venting for dryers, using 4" rigid, is 25' max with 5' subtracted for each 90. So, despite knowing this, it seems my only option is an 18' horizontal run which drops down from a dryer box ( to 1st 90), with a straight shot through the joist bay and out the soffit/eave (second 90). I'm even planning on having it slope downwards about 1/8" per ft of the run. I don't have a dryer yet, so can't check the specs, but do you think that this ducting setup (although about 3' longer than the 15' allowable by the mech'l stds) would cause a problem? Booster fans are expensive, and I really see no other way to run it. I'm looking for your real world experience, here. Your insight is much, much appreciated. :)

Don't really see too much of an alternative if you have to run the duct that way. Try a dbf110. I have an outlet for those and although not real cheap, $206 isn't too bad considering the headache it will save later.

Check your local building department. Because, you may/not be under the 2006 code which says no booster fans and follow manfacturer's duct length, longer may be accepted. No screen at termination, no screws, protect all duct with-in 1-1/4" from surface with a steel plate. Be sure to insulate and vapor barrior wrap the ducking in attic after foil tapeing all joints, even elbows.
Be safe, Gary