Managing the budget: financial insecurities

My parents taught me how to handle money. I spend it wisely, I don't buy stuff I cannot afford and I am super good at saving. I don't live on the edge. Now that I have my own lab, I notice that I am handling my lab budget in the same way. I want to make sure that we have enough money for experiments, which are always more difficult to interpret than planned and which always require more follow up than the aims section in my grants promised.

In a recent comic I read at the Node (originally from the Journal of Cell Science), Mole suggests keeping a monthly budget: if this month's funds run out, people can no longer order. They can read instead. Fortunately, my peeps read papers on their own and the only one who should be forced to spend more time reading the literature is me, alas.

I am slightly less strict than Mole (I must confess that I am also never sure whether Mole's advice is to be taken seriously - could someone fill me in on that?): I check throughout the year (every three months or so) to make sure that as a group we don't overspend on an annual basis. I know what that number is, because I do allocate a virtual amount of money to spend on reagents as well as on mice in a spreadsheet where I keep tabs of all my grants.
It was difficult to decide what strategy to follow at first. Both during my PhD and postdoc I was in the lab of established scientists who were pretty well off. They always had a bit of money here and there. I was hardly ever told not to order stuff (at least not for financial reasons - one of my PIs did have the occasional nasty habit of not signing off on orders if he didn't like the experiments you were planning to do). My rich mentors never taught or told me anything about budgeting.
Starting out on my own, funds were considerably more tight and, being the goody two shoes that I am, one of my worst fears was to overspend and to run out of money. At the same time, I didn't want my people to suffer from joining a small, junior lab. They shouldn't feel like they cannot order stuff when they have an exciting new idea. So I am educating my lab members like my parents educated me: Think about what you need before ordering, make sure we don't buy stuff we already have, try to find the cheapest price, but if you really need it: get it. So far, we are doing okay.

My biggest concern now is to decide how far I can stretch my grants when it comes to hiring new people. I am now facing a situation where I got a few "smaller" grants* and I am not sure what to do. The "problem"** is that this grant only pays for, say, half a PhD. In my ideal world, I would save that money until I would get another grant that would pay for the other half, but that's not how it works. The granting agencies want you to start spending that money within the fiscal year (so their bookkeeping checks out), but I am just not comfortable hiring someone when I am not sure I will have money to keep them for the second half of their contract as well. My risk averseness doesn't really serve me well here. I can already imagine the sleepless nights and the stress that is going to bring me. So how do others deal with this? Should I just get over it? Do others ask the department to tie them over in case something doesn't work out in the end? Any good or bad experiences are welcome in the comments.


* it's funny how quickly you start to think about a certain number as "small" simply because you are now also dealing with amounts of money you only used to hear about in the Powerball lottery.

**its also funny how having money can be as much as a problem as not having money, just a different kind of problem.