Per Pathfinder: "...he may make one additional attack using any combination of unarmed strikes or attacks with a special monk weapon as if using the Two-Weapon Fighting feat. For the purpose of these attacks, the monk's base attack bonus is equal to his monk level."
At 8th level he gains the equivalent of Improved Two-Weapon Fighting, and thought it isn't mentioned in the text at 15th level he gains a third iterative attack with the off-hand (you can see this by examining the table that shows the progression of Flurry) so he gains Greater Two-Weapon Fighting at 15.
To apply the Trailblazer rules to it (remember the attack bonus is at a -2 penalty from the TWF, and when iterative attacks begin to appear attacks are at a -2 penalty when the BAB reaches +6, -1 when the BAB reaches +11, and the penalty is gone when the BAB reaches +16)...
Lev 1 : Main -1, Offhand -1, TB equivalent -1 & -1
Lev 2 : Main +0, Offhand +0, TB equivalent +0 & +0
Lev 3 : Main +1, Offhand +1, TB equivalent +1 & +1
Lev 4 : Main +2, Offhand +2, TB equivalent +2 & +2
Lev 5 : Main +3, Offhand +3, TB equivalent +3 & +3
Lev 6 : Main +4/-1, Offhand +4, TB equivalent +2/+2 & +4
Lev 7 : Main +5/+0, Offhand +5, TB equivalent +3/+3 & +5
Lev 8 : Main +6/+1, Offhand +6/+1, TB equivalent +4/+4 & +4/+4
Lev 9 : Main +7/+2, Offhand +7/+2, TB equivalent +5/+5 & +5/+5
Lev 10: Main +8/+3, Offhand +8/+3, TB equivalent +6/+6 & +6/+6
Lev 11: Main +9/+4/-1, Offhand +9/+4, TB equivalent +8/+8 & +8/+8
Lev 12: Main +10/+5/+0, Offhand +10/+5, TB equivalent +9/+9 & +9/+9
Lev 13: Main +11/+6/+1, Offhand +11/+6, TB equivalent +10/+10 & +10/+10
Lev 14: Main +12/+7/+2, Offhand +12/+7, TB equivalent +11/+11 & +11/+11
Lev 15: Main +13/+8/+3, Offhand +13/+8/+3, TB equivalent +12/+12 & +12/+12
Lev 16: Main +14/+9/+4/-1, Offhand +14/+9/+4, TB equivalent +14/+14 & +14/+14
Lev 17: Main +15/+10/+5/+0, Offhand +15/+10/+5, TB equivalent +15/+15 & +15/+15
Lev 18: Main +16/+11/+6/+1, Offhand +16/+11/+6, TB equivalent +16/+16 & +16/+16
Lev 19: Main +17/+12/+7/+2, Offhand +17/+12/+7, TB equivalent +17/+17 & +17/+17
Lev 20: Main +18/+13/+8/+3, Offhand +18/+13/+8, TB equivalent +18/+18 & +18/+18
Putting it all together...
Lev 1 : -1/-1
Lev 2 : +0/+0
Lev 3 : +1/+1
Lev 4 : +2/+2
Lev 5 : +3/+3
Lev 6 : +4/+2/+2 (recommend +3/+3/+3)
Lev 7 : +5/+3/+3 (recommend +4/+4/+4)
Lev 8 : +4/+4/+4/+4
Lev 9 : +5/+5/+5/+5
Lev 10: +6/+6/+6/+6
Lev 11: +8/+8/+8/+8
Lev 12: +9/+9/+9/+9
Lev 13: +10/+10/+10/+10
Lev 14: +11/+11/+11/+11
Lev 15: +12/+12/+12/+12
Lev 16: +14/+14/+14/+14
Lev 17: +15/+15/+15/+15
Lev 18: +16/+16/+16/+16
Lev 19: +17/+17/+17/+17
Lev 20: +18/+18/+18/+18