I have an old Trek 830al that I have had new since 99. I have come across some used parts that I am swapping over to the 830al. So I am doing a complete tear down and rebuild, cleaning every thing really well. I currently have a front Derailure that is mounted using the bottom bracket. I currently have this bottom bracket

In the 73x121 form. Since the Old BB is toast, I need a new one and I have a clamp on front Derailure and so I don't need the bottom bracket that holds the derailure. So my question to you. Is how much shorter do I need to get my spindle for the new bottom bracket. I happed to have a 68x107 that i screwed in there I know its not the right shell with. But the spindle lenght seemed about right. Is the difference between 110 and 107 just guessing?