I have an SEO doubt: Should I link back to the index page or services page? If so, what should the ratio be? 50-50, 25-75, etc.
You can submit both to get rank and there is no fundamental ratio to submit them. You just have to submit in quality directories and fast approval as well.
When it comes to inbound link building you should build links to all our web pages; naturally backlinks usually point to an internal web page. There is no golden ratio, but you should try to make it 50/50. Meaning 50% of your links go towards your root domain and 50% go towards your internal pages. Make sure to mix up the anchor text and use some naked links to make it appear more natural.