The default MTU is 1500 and if you expeircne any issue with default MTU ... e.g. MSN /Yahoo mail not logging in or any site login issue. You can chnage the MTU to 1496.
I will try some math... The maximum Ethernet payload MTU is 1500. Let's fill all cells with data and avoid padding. 48 * 31 = 1488. One thing you are missing is that the last cell will have 8 bytes trailer (RFC 2364), so we need to reduce our data chunk by 8 bytes 1488 - 8 = 1480. Finally, 2 bytes PPP overhead gives us 1478 MTU. That's assuming PPPoA with IP over AAL5/VC-MUX.
Now, if we look at PPPoE bridged over ATM, we need to consider Ethernet frame overhead that also should be placed into ATM cells. So, from the above 1480 we take out 8 bytes PPPoE overhead (6 bytes PPPoE and 2 bytes protocol Id), then additional 18 bytes of Ethernet overhead and now we have 1454 (one is out of the way). What did "bright" DSL specialists do next? They said: "Ok, since PPPoE has 8 bytes overhead and that overhead does not apply to PPPoA let's simply add those 8 bytes to our payload" and that's how they came up with 1454 and 1462. That, however, makes no sense because for PPPoA calculations they overlooked the fact that 18 bytes of "used to be" Ethernet overhead no longer there and they also forgot to subtract 2 bytes of PPPoA PPP overhead. PPPoE calculations are also questionable because of in case of LLC (vs VC-MUX) which is normally used for PPPoE there are further 8 bytes of LLC overhead (RFC 2684).
But to answer your question about most optimal setting from perspective of minimising any ATM padding ...
1478 for PPPoA/VC-MUX
1470 for PPPoA/LLC
1446 for PPPoE over ATM/LLC
Hope this makes sense.