Welcome to routing!
Well this bosh machine looks good!
but it is european and then it is metric, see about guide bush here: Accessoires Grand Public Bosch
Sorry that' s french but you can see all is metric here.
If i where you, i would not modify this guide bush, because you can also get good
things that are in metric, and that it is not so easy to sand down 0.3mm of steel
it will also weaken this expensive guide bush, and make it rust.
But I would rather fit a sub-base under the router that will improve stability when using dovetail jig.
And then use some a new guide bush that fits the sub base.
There are such kits from quite a few makers and they are very usufull.
Don't know much about what the shopping market is in your area but heres samples: http://www.trend-uk.com/en/UK/produc..._subbases.html
And a few set of guides: http://www.trend-uk.com/en/UK/produc...de_Bushes.html
This might be intersting: http://www.trend-uk.com/en/UK/trend/...bility&id=1154
This is quality tool , but many other quality tools on the web.
By the way most of dovetailling kits comes with specific guide bush and cutter(s).
You must be sure to be using the good cutter type.