Using Projects for Time Allocation

I'm guessing we're not the only company with this challenge, but we're in our infancy in using proper project management within our organization. Curious if anyone has advice on how to deal with a certain scenario.

-We know roughly how long these tasks should take, and their interdependencies by the individual's estimations, but we're hoping to 'stretch' the project durations based on the fact that one person can't do three four-hour tasks at the same time
-In addition to the above fact, the individual can only give roughly 30% of his time to this specific project, based on juggling a bunch of things at any time, so we would need to extend the presumed durations of these projects to account for that

My initial thoughts are that we could do the following, but I'm curious if anyone has a better idea, as these seem like workarounds and aren't that elegant for reporting on:

1. If there are unrelated tasks all to be done by the same individual, add dependencies so they're never scheduled in parallel
2. Extend by factoring in the allocated percentages manually (i.e. if a task is expected to take 3 days, but the individual only has 30% of his time available for it, then (3/0.3 = 10 days)

Thanks in advance!

Ben