The way I would do it, is to use a Custom Modifier (Add Modifier -> Custom Modifier (at the botttom) and then dial in the level you want (+/- 1/4, +/- 1/2) and then edit comment field with the proper endurance.
Printing it out on a character sheet will require some decisions. The tag POWER_END is the tag that displays the regularly calculated value. So you would have to edit the export file to remove that tag (but then you loose all the END cost listings), or you'd have confirm the comment field prints (it did on the one I tested), and make sure your players use that value.
You can also create a Custom Power and set all the properties, but use the 'Define Custom END Column Output' and set the amount you want displayed. You would still apply Modifiers as usual then. This method has a few more steps, but the END cost is picked up by the PDF and export templates normally,
Those are two ways I know it could be done, if there's a different/better way, I'm sure someone will post it