Doerdich:
The formula Fide for variation of rating is:

NR = OR + (SG - SW) * K

NR is New Rating
OR is Old Rating
SG is Score Gotten
SW is Score Waited
K is a Constant

K can have three values :
K=25 when the player hasn´t finished 20 games
K=15 when the player has finished 20 games and his rating is lower than 2400
K=10 when the rating of player is upper than 2400

Score Waited is calculated with a list of percentage, but you can calculate approximately in this way :
SW ~= 0.5 + [(RP - RO) \ 800]
RP = Rating Player
RO = Rating Opponent

The highest value of [RP - RO] is 350 and the lowest is -350. If the different is higher than 350 or lower than -350 then we have [RP - RO] = (+-)350

All this means that the highest variation can be:
Rating Player = 2000
Rating Opponent = 1500
Different above to 350
Score Waited ~= 0.5 + [(2000 - 1500) / 800] =
= 0.5 + [500 / 800] =
= 0.5 + [350 / 800] =
= 0.5 + [0.4375] =
= 0.9375
If the player rated 2000 loses the game then
New Rating = 2000 + ( 0 - 0.9375 ) * K =
= 2000 - 0.9375 * K
With K=10 => New Rating is 2000 - 9.375 = 1990.625
With K=15 => New Rating is 2000 - 14.0625 =
1985.9375
With K=25 => New Rating is 2000 - 23.4375 = 1976.5625

The highest variation is (+-)23.4375 for players with rating lower than 2400 and no more of 20 games and for strong players is only (+-)9.375