Ask HN: A Vesting-Over-Time Scheme for Startups

To give some background, I founded a startup this January, and since have gathered three others as Cofounders. Since money is not involved yet, we need to figure out a way to share equity fairly among the team members. * I am thinking of a vesting-over-time scheme with a Monthly Schedule, and wanted to run it by the HN community and get their opinion regarding this as well. The scheme works as following. The total ownership pie will be first divided in to two * Fraction of fixed ownership (original founder, investors, etc) * Fraction to be vested by the team dynamically The fraction left for the team is to be vested and divided based on each persons contributions on a monthly schedule. At end of every month, everyone would agree on the work contributed by them (for example 50% by #1, 25% #2, 25% #3, etc). By having a log of this every month, we can calculate the total work contributed by a team members as a fraction. This would account for anyone who joins in later on, who works full time, works part time, who go the extra mile, and anyone that leaves (as some of the cofounders have obligations such as school and other work after the Summer). So end every month, the part of the startup's ownership is redistributed. My goal is to make the scheme fair for everyone. The fixed ownership is owned by by someone such as a investor and does not change over time. For example, if we agree to give an investor 10%, that would be added to the fixed ownership fraction (and that 10% will come off the team's ownership fractions in a fair manner). What do you all think about this scheme? Any pitfalls? Good/Bad? Any Alternatives? Here is a spreadsheet with this. There are charts on the last three if nothing I said makes sense... The Numbers are hypothetical just to to be used an example. http://spreadsheets.google.com/ccc?key=rBkybK6z5iSGmpfU-EWOyzw