In the Character Selection menu the displayed Magnet value only shows the percentage applied at the start of the game instead of the actual value. Also, the shown percentage doesn't increase exponentially, which is incorrect. These two factors combined make it difficult to tell how much Magnet you'll actually have at the start.
The Magnet value shown in the stats panel, both in the Character Selection menu and in-game, is rounded to the closest whole number. The actual value of Magnet is a decimal number.
To calculate Magnet initial in-game value using the value shown in the Character Selection menu is a little difficult as each egg bonus adds 1% to it and, in fact, its not a multiplicative bonus, but a addition in the base value. So, to do it you'll need to use the following formula:
Magnet = (((Menu%Value - Character%Value - PowerUp%Value) * 0.3) + 30) * Character bonus * PowerUp Multiplier

For example, Poe with no Eggs Bonus:

Magnet = (((75 - 25 - 50) * 0.3) + 30) * 1.25 * 1.5625 = 58.59375

Note that 0.3 is the value added by the Golden Egg in the Magnet base value, so the purpose of the formula is to find the bonus acquired with Golden Eggs using the % showed in the menu as each one increases it by 1%, to sum it with the base value as showed in the previous formula and then to multiplying by the character and PowerUp bonuses.