Seems after long discussions that there is no direct equivalent?! "Pi mal Daumen" means for me primarily that you don't measure something accurately but take a bit of a gamble with lengths, sizes and other dimensions. In the UK at least, the phrase "Pi mal Daumen" has no equivalent because is superfluous, as any DIY or professional construction or refurbishment work is done in a slightly haphazard way anyways. ;-)
And the Brits are even proud of it :-/ no offence!
Just my non-representative observations as a German engineer after three years in London...
If you want to say that you estimated a figure you don't know exactly (or can't be bothered to determine exactly), "roughly" or even more vaguely, "ballpark" is suitable.
If you refer to your way of doing manual work, you could say "i'm taking approximate measures" or "i'm working with rough measures".
Und #21: Der Stegreif hat nichts mit stehen und greifen zu tun, so logisch das klingt, sondern ist der Steigbügel eines Sattels. "Aus dem Stegreif" bedeutet, daß ein Reiter nicht absteigt um eine bestimmte Aufgabe zu erledigen, er macht sie "aus dem Stegreif", siehe http://de.wikipedia.org/wiki/Stegreif