I find the best approach is to temporarily modify the node graph when exporting the LUT.
Resolve bakes in a transform from the working space, the RRT, and the current ODT (but not the IDT) when exporting a LUT. So in the OP’s situation you get a LUT which transforms from DaVinci ACES to Rec.709. This is not useful for several reasons. It doesn’t work if applied in the node graph in place of the grade, because it already includes the RRT and ODT so those get applied twice, giving the wrong result. Also, because DaVinci ACES is a gamma coded space, not log, the working space has values greater than 1 in it, and those are not handled by LUTs (unless you include a range or shaper, which Resolve’s “Export as 3D LUT” does not) so anything more than 2.5 stops above mid grey will be clipped by the LUT. This is bad!
DaVinci ACES is also non standard, so does not work well outside Resolve. I believe it has been removed entirely in the Resolve 14 beta. I recommend using ACEScc or ACEScct, the latter being my personal favourite.
To produce a LUT which can be used in Resolve in place of the grade, or in a compositing app, you need to remove the RRT and ODT from your exported LUT. Even if you choose “None” as your output transform, Resolve will bake a transform from the working space to linear ACES into the LUT. You therefore need to set an output which is the same as your working space. In Resolve 14 you can choose e.g. “ACEScct” as the output transform, so exported LUTs should then be able to be imported and used in place of the grade. I am not yet running Resolve 14, so have not personally tested this.
In Resolve 12.5 you cannot do this, so need to temporarily add a transform to your node tree to invert what Resolve “is going to do”. You can do this with DCTL. Pardon the self promotion, but I sell a set of DCTL files (ACES Utility Conversions) which can be used for this purpose. I also made a short screencast* (sorry it’s a bit rough and ready; I must make a better version!) showing an example of using the DCTL to export a LUT.
*Note: If you are confused by my reference to the “bubblegum effect” in the video, I am referring to an effect that happens with ACEScc, which I describe in this Mixing Light article.
If you want to export a LUT which includes the whole ACES process, IDT, grade, RRT and ODT, you can also use one of my DCTL IDTs to do this, by setting Resolve’s IDT to “None” and adding a DCTL IDT to the node tree, so it is included in the exported LUT, which you can then use stand alone, e.g. in a LUT box on set. I describe this process in another thread.