W208 CLK underbonnet earth point for jump starting

[tjamesbo]

Yesterday evening the CLK wouldnt start after a brief charge (1hr ) still no start so checked the battery using my TOPDON battery tester which showed 10.3 volts and a replace diagnostic however However i needed to move the car and therefore opted to use my Ring 3000 Jump start pack to start it and move it . As it was difficult to get a good contact on the terminals in the boot i thought it would be easier to use the typical mercedes posts under the bonnet which are there for jump starting purposes ( or charging if your locked out of the boot )
Theres a positive post but no negative post and an absence of any other earthing bolt points anywhere in proximity and as the jump starter only has short cables designed to go on the battery i was back to the boot battery terminals and it did make me wonder ;
1 is there any difference jump starting from the boot battery or the engine bay post ?
2 could i use the metal bracket for the positive post as an earth as the bracket is bolted to the body (see picture )
3 or should i add a jumper cable to the short ring lead and then the other end to a suitable earth (not good a good idea i think with the current involved )
4 should i depaint a wing bolt next to the positive post and then its easily available for future use (although hopefully with a new battery it wont be required any time soon )

 

[BachgenDrwg]

Too be honest any body piont will do but if there is an engine lift eye that's preferable as its bulky and has a short route to the starter. If your battery is stuffed it will be sucking up the charge meaning less power to the starter. Direct to the start point and a good earth near the starter is always best when jumping.
T

 

[alexanderfoti]

That cable should only be used to inject 12v to open the boot etc.

The cable is too thin to support a jump, but as you say , you can use ANY unpainted metal. Engine block, small bracket you mention.