The "72 xp needed" means that it will take another 72xp to upgrade the skill to the next percentage point. So if your skill was at 11%, then it would take 72xp to get the skill to 12%
Too increase the 1st skill perk to 100% from 0%, you will need a total of total of 210,484 XP.
The World of Tanks wiki describes it in detail:
"It is important to recognize two facts about the amount of XP required to increase Training Level.
First, the total amount of experience required to achieve a 100% Training Level in the 1st Skill or Perk is double that required to increase the Major Qualification to 100%. This doubling occurs each time with each new Skill or Perk. Training the 2nd Skill or Perk to 100% requires double the amount of XP that the 1st Skill or Perk requires, the 3rd requires double the XP of the 2nd and so on. To increase the Training Level of the:"
Major Qualification from 0% to 100% requires a total of 105,242 XP.
1st Skill or Perk from 0% to 100% requires a total of 210,484 XP.
2nd Skill or Perk from 0% to 100% requires a total of 420,128 XP.
3rd Skill or Perk from 0% to 100% requires a total of 840,255 XP.