I'm not sure you're using the correct 'r' rune. In the inscription on Balin's tomb, Tolkien uses #12 rather than #29 (referring to the Angerthas table in Apendix E of the LotR). Same for your 'n': on Balin's tomb, #22 is used. You're using #12.
For 'mb', I'm pretty sure #7 should be used, instead of #6#2.
You've got your 'u' and your 'ú' the other way round. Again, refer to Balin's tomb - it has examples of both.
As Mixxiphoid points out, you're missing the 'an' in 'ambartanen'.
Not sure if there are any other issues.
Also, there's no reason why you need to use the Certar rather than the Tengwar. Both were in use during the First Age. The difference was in their use:
The Tengwar were devised for writing with brush or pen [..]. The Certar were devised and mostly used only for scratched or incised inscription. (Tolkien, LotR, Appendix E II)
This is how I would do it, I think: