The same level of it's lower leveled Pokémon is the standar.
Medium Slow Pokémon have different EXP curve, make the proper ajust with them. Level 31 Dugtrio and few other exceptions (like gift Pokémon) should be properly ajusted.
+3 if Starter, until Brock, 2+ until Misty, 1+ until Lt. Surge, 0 later.
+1 for each less Pokémon from 6 on the Party (Full Party = 0, 3 Pokémon Party = 3+, 1 Pokémon Party = 5+)
+1 for Fast (EXP) Group Pokémon
-1 for Slow (EXP) Group Pokémon
-2 if Fast Play, until Brock, 3- until Misty, 4- after Misty
-4 if Speedrun, until Brock, 5- until Misty, 6- until Lt. Surge, 7- until Erika, 8- after Erika
Brock: Level 10
Misty: Level 18
Lt. Surge: Level 21 (Pikachu is underlevel)
Erika: Level 29 (Tangela is underlevel)
Sabrina: Level 35-37
Koga: Level 35-37
Blaine: Level 40
Giovanni: Level 42
Elite Four: Level 47
Lorelei's lower level is 53, but there's not too much experience from defeating Giovanni to reach to the Elite Four, so, the difference of levels is just of 5 at best, you'll need to grind or use few Pokémon if you're intending to reach the benchmark of level 50. Level 35 of Sabrina/Koga if you're facing one of them first, level 37 for the second. You may want to grind a bit for the Yellow Version against some like Sabrina or Lt. Surge if you're not carrying the specific counter.
The for each Pokémon on the party thing is more precise once you reach to Lt. Surge, before it's more like the half of the effect, while it's almost the double of the effect once you're on the nearly-end part of the game (entering to the Giovanni's Gym, for example), it's more like a curve.