Calculate Latitute values based on tiles used

[GLS]

Hello,

I am reasonable new to the 'mapping' world, and I am strugling with a problem.

Based on Tiles used in MobAC, I am trying to calculate intermediate latitude point, based on known points.
I was trying to do this on the tile numbering, give by MobAC.

My process is to define one area, with several Zoom levels in MobAC. The highest level is level 7. From this level, I do know the North, West, East and South values. The so created XML-file defines the tile information for all specified zoom-levels.
Some of the lower levels (8 - 11) will need to be split into smaller parts.
So (eg.) on level-11, I do know the coordinates of the area selected. I also do know the tile numbers, of the whole area and for the smaller parts.

However, when I do calculate the coordinates of the intermediate parts, I do get a small difference.

Is there anyone, who is able to tell me, if there is a way to prevent these delta's?
And if there is a way, how do I use it?

Thanks on forehand,

Attached are 3 files:

• 1 screen dump of level-7 which can be loaded in one piece,
• 1 screen dump of level-11, which needs to be broken down in 4 parts
• 1 screen dump of an Excel simulation, showing the tile numbers, and degrees. The known information has
  been shown in 'underline'-text.
  The (wrong) calculated values are shown in Dark-yellow background.
  The MobAC retrieved coordinate value are shown in green background, Italic.
  The measured delta's are shown in Red-bold text.

PS, on Longnitude calculations, it goes fine without any problem.