I am installing a 200 amp panel on back of house fed by meter and disconnect mounted on pole. Plan to run 4/0 aluminum wire maybe 80 to 90ft. Plan is to bury schedule 80 pvc. Charts show 3 inch pipe for 200 amp but boxes in store only have knockouts up to 2.5 So do I reduce at the box?
I have no idea what you're looking at, but no, the NEC does not demand a pipe that fat for a 200A feeder
I have absolutely nary a clue what sources you are looking at, but according to Chapter 9, Table 5A of the 2017 NEC, a 4/0 Al XHHW-2 compact stranded conductor takes up 176.3mm2 of fill. Multiplying that by four, which is quite conservative as you will see, gives us 705.2mm2 of fill used by the entire feeder, whereas a 2", Schedule 80 PVC conduit has 742mm2 of usable fill area in it as per Chapter 9, Table 4 of the 2017 NEC. So, clearly, a 2" conduit will suffice according to the NEC; you can upsize to 2.5" if you wish for ease of pulling, but no, you don't need to faff with reducers just to get your conduit to mate to your box.
Note that in all likelihood, you will not be running 4 4/0 wires for this. The largest Mobile Home Feeder (MHF) quadruplex available is a 4/0-4/0-4/0-2/0, and there's also a 4/0-4/0-2/0-4 configuration available; while one could get 4/0-4/0-4/0-4/0 URD, there's no reason to given that Code permits a smaller ground wire and cables with the smaller ground wire are readily available in URD and MHF styles. If you're actually pulling individual wires, a typical bundle would use 4/0 XHHW-2 Al wires for the hots and neutral, with a 6AWG bare or green (THHN) copper grounding wire in the conduit.
We don’t calculate conduit fill based on lookup tables. We calculate it based on the actual size of the wires, or in the case of an oval cable, the swept area as it twists (like a circle of the large dimension).
We total up the effective cross-section area of the wires, and then, compare that to the actual cross-section area of the pipe in question - and warning here, that’s a serious “snag” where Sched 80 PVC is concerned, since the actual inside diameter of Sched 80 PVC is smaller than its trade size.
The area of the wires must be less than 40% of the pipe when 3 or more wires are present.
Someone may have computed that table “for you”, but they may have gotten it wrong, or may be factoring for other things than you are. You said “60% of fill” well that’s not allowed, but if you mean “60% of the max allowed”, that is fine, and wise.
By the way, it simplifies the math to not have to keep multiplying and dividing by pi. That’s why they invented a unit called “circular mils”, often seen in wires larger than 4/0. A mil is 1/1000 of an inch, so a circular mil is 1/1,000,000th of a “circular inch”. A circular inch is a circle 1” in diameter.
“thousands of circular mils” is called “kcmil” or “MCM”, the latter uses the old Roman style of M for thousand. This unit is widely seen in wires larger than 4/0. But in that case, that only counts the metal part (216 kcmil for 4/0). For conduit fill, we need the size of the insulation (384.4 kcmil for 4/0 XHHW).
For instance a 0.4” (400 mil) diameter wire is 160,000 circular mils or 160 kcmil or 0.16 circular inches.
Jacketed cable must be treated as 1 wire of the large dimension. Unjacketed wire like MH feeder, you are free to count the area of the individual conductors only as if they were separated.
However, know that any time multiple wires are bound together, they will be much stiffer than an individual wire. That makes the pull tougher. You can compensate with bigger pipe, but the DIY friendliest solution is multiple individual wires.