If this result is less than 10, the power continues to fight. If it is exactly 10, the power will sue for peace, but can not accept an unconditional. If the roll is great than 10, the power will sue for peace and may accept unconditional if the player deems it wise. On a roll of 15 or more, the power must sue for peace and accept whatever is offered. Unlike DoW's, you do not check for peace every turn, or even every turn in which a power is at war, rather, surrender checks are made when certain event come to pass.
All modifiers are cumulative.
With these rules, it should now be possible to have a reasonable judgement of when a country has had enough and sues for peace. If you decide to try them out, let me know how they work and any suggestions you may have.