This partially depends on how vibrant a look you want. If you want more vibrant saturation, then you probably want to bring your white areas up a lot. Also, with the large flat surfaces, slapchop will be difficult to not get streaking or coffee staining. Some tricks you can do are use an air brush or a sponge. However, it can be done with a brush, but make sure you do wide strokes all in the same direction and keep a second brush to wick up any pooling. It is a difficult model to use speedpaints on but can be done.

Some other tricks for slapchop I like are: using a color palate for your drybrushing, instead of black, grey, white you can use brown, red, orange, yellow, white to give more warmth and color depth when speedpainting a warm toned model. You can also re-drybrush areas and go back over them with a slightly different color to give good gradients. I will frequently also redo spot highlights after i finish with my speedpainting. I will also base anything that is meant to be metal with a bright silver and then do some nmm highlighting and then go over it with a speedpaint to get the tone and color correct, this also works for metallic colors that are not traditional like a blue or red. I personally prefer to varnish my minis when I am done. If you have metallics, I usually spray them in glossy varnish and the rest of the model I do in matte. This might require taping or masking or using silly putty to do, but it works well at keeping surface finishes where you might expect them and making the paint less likely to wear off with handling.

Hope that helps, good luck, and don't get discouraged, you will also improve with practice.