Sure. As a very rough gauge, adding +1 to the DC will drop the players win rate by 10% for a complexity 1 and around 15% for a complexity 3. Be cautious though, a +1/-1 here or there should be fine, but even a +2 can make quite a difference.
Also, keep in mind that my tables average a party's skill number through 1-3 level, 4-6th, etc. There 1st level numbers will be lower than the average, and there 3rd level slightly above. With that in mind, what I would do is use the system as is at 1st level (as the win rate will drop a bit and be more into your desired range). Then by 2nd level it will jump up a bit. By 3rd level, add in your +1 to DC to get the win rate back the way you like it. Then at 4th and 5th level drop your +1 to DC, and add it back in at 6th level, etc etc.
In addition, if you want just bump up the complexity 1 notch to make the challenge harder. One advantage of my system over the base system is that higher complexity ALWAYS equal a harder challenge. Using a complexity 3 instead of a complexity 1 will lower the win rate by about 10% using your standard numbers.