The experience you need to gain a new level is calculated following way:

GAINING EXPERIENCE

As you gain experience, you will rise in stature within your guild. The amount of experience needed to gain a level is calculated this way:
1) Determine the Base XP for a level 1 character of your race and class. This is equal to 1500 + class penalty + race penalty. Call this Base XP.
2) Determine 20% of that amount. Call this the Level Adjustment.
3) The amount you will need to get to next level is: ((Current Level - 1) * Level Adjustment) + Base XP

Example 1:
A Human Warrior will have a Base XP of 1500 (1500 + 0 + 0), and a Level Adjustment of (1500 * 0.2) = 300xp.

To move from level 2 to level 3, the human warrior would need to earn ((2-1) * 300) + 1500 = 1800 xp.

To move from level 31 to level 32, The human warrior would need to earn ((31 - 1) * 300) + 1500 = 10,500 xp.

Example 2:
A Storm Giant Paladin will have a base XP of 2400 (1500 + 500xp racial
penalty + 400xp class penalty) and a level adjustment (2400 * 0.2) = 480 xp.

To move from level 6 to level 7, the storm paladin will need to earn
((6-1) * 480) + 2400) = 4800 xp.

What the plus there(bolded in the following formula) means is that your new tnl will be:

tnl after mobdeath = ((Current Level - 1) * Level Adjustment) + Base XP + 100*(Current Level)