Steve,
We just added a new execution algorithm called "Spend-X". (You can probably guess that no one on our team majored in Marketing!)
This trade-type allows you to do what you mentioned in the original post. You also have the options to not enter limit price and let the algo calculate purchase quantity at market price. It is fun to watch it squeeze in every possible Satoshi within the target spend amount.
Please check it out on Camp BX and let me know your thoughts.
Thank you,
Keyur
From the draft FAQ:
Spend-X Algorithm allows to you spend a desired amount by calculating the purchase quantity on your behalf. These calculations are based on current orderbook prices, and commissions are factored in automatically.
To use Spend-X trade, enter the total amount you want to spend in "Total Amount" field. After this point you have two options:
First option is to leave the "Price" field blank, and let Spend-X algorithm purchase Bitcoins at Market price. Or,
Second option is to enter desired limit price per Bitcoin in the "Price" field. Spend-X will calculate quantity after commissions.
That's it! Execute away.