In slic3r printer configuration, add this to the "before layer change gcode":

; before layer [layer_num]

This will print the layer number, as a comment, between each layer in the gcode, so you can find them clearly.
(You could search for "G1 Z2.0" or similar, but this may not be what you expect depending on layer height settings, offsets, autolevelling adjustments, etc.)

You will know from the preview (in RH, etc) what the layer count will be (I think it prints in the end of the file somewhere too...), so you can calculate approximately which layer number is about 3% or 30% through the print - e.g. 3% through a 200-layer print is about layer 6, right?

Now doing it manually is not too hard for the odd tricky long print - search through the exported gcode (in repetier host you could just do it in the viewer, and even if you don't save it to a file it should run as-written when you press print) for the layer you want to change the temp at, and insert the appropriate "M140 S55" etc. gcode as needed.

If you are handy with some kind of scripting language, you could create a post-processing script that does the above, and slic3r can be configured to run the gcode through it - I guess from your description of not being a 'g-code writer' this is probably a bit of a long shot! Maybe something fun to muck around with though.

M140 S0                     ;heated bed heater off (my current setting)

anyone know how to reduce heat bed temp during print, at a certain percentage (say 3%) of completed print. Then repeat, to reduce again, say at 30% ??

Right now I can use my tuning control to reduce, but I cant leave it..like overnight.

Thanks