The construction rules do work, if you think through the implications. There are two different numbers involved:
1) The duration of the project, in calendar days from start to finish
2) The number of downtime days that the PC has to spend on-site, supervising
The PC has to be prepared to spend at least as many downtime days on-site as the days quoted in the table. If not, the project will never finish. That's the minimum cost, in downtime days, to the PC.
If he stays on-site for the whole project, it will be finished in that many days. No problem.
If he takes days out to do other things (either in one block or in odd days here and there - it doesn't matter), both the duration and the cost to him in total downtime days will increase.
For every day out, the duration increases by 3 days and the cost in downtime days increases by 2.
Example
You start to build something with a book cost of 30 days but during construction, you take 5 days out carousing.
The duration of the project increases to 45 days (30+3*5 = 45).
The number of days you must spend on-site increases to 40 days.
You can calculate that as days present = total duration less days absent (45 - 5 =40)
You can also calculate it as original cost + two times the number of days absent (30 + 2*5 =40).
That's how the rules work, as written. Of course, as DMs, we are free to change the rules if we don't like them. For informal play, PCs essentially have an unlimited supply of downtime days and spending them merely delays the next adventure on the campaign calendar, unless we have scheduled timed events to interrupt them. For organised play it's a bit different because downtime days are awarded in limited amounts at the ends of adventures and players may have to save them up until they have enough for whatever project they have in mind.
