65% is not bad. Most recipes only expect 70%, so you're not going to be that far off to begin with if you are getting 65%. I wouldn't do partial mash unless you want to. Use Beer Smith or BrewTarget and just adjust your recipe for your efficiency. Read up on how to calculate efficiency first. Understanding your volumes and gravities at each step will help you improve. If you really want to adjust your gravity, 1 lb of DME will add 9 gravity points to a 5 gallon batch. I would just use pale DME; it's not going to affect it much, but if you use amber or dark it may.
I should add.. the simple technique of adding grains to water (instead of water to grains) will make it easier to avoid clumps and dry spots. Stir the crap out of your sparge as well as your mash, and vorlauf both the mash and the sparge. Those techniques added about 2-3% each to my efficiency, and I'm getting somewhere around 75-80 pretty consistently with a very rudimentary batch sparge setup. I use brewtarget and pretty consistent with volumes and efficiency, with the caveat; lower water to grain ratios and higher gravity beers reduce efficiency and vice versa.
Also, lower water to grain ratio will hurt your efficiency, but may be required for style (mouthfeel and maltiness), so better to anticipate a bit lower efficiency in that case and adjust accordingly, rather than sacrifice style for efficiency. Higher ratios, like 1.5 will improve efficiency, but will make for thinner beer.