I don't want to be Devil's advocate again.. I hate it.. but one of the commonly used tags is follow, noindex.. so if the spider passes through the link to your website but the link isn't added to the index.. Google knows about it and its power already.. why wouldn't it pass link juice? Why would it have a smaller power than a link you tag as nofollow so you express your will for google not to check what's behind it, but google still does..
Ok, first off, nofollow and noindex are 2 different tags with very different use cases. Noindex tells Google to drop a page from the index. Once a page is dropped, any calculations (link juice that you call it) in the algorithms that took that page into account will change. Any page that has a link to/from that dropped page will be reduced in power (or become stronger if the dropped page was shit). This is completely in line with that I said above: if a link is in the index, it will be taken into account. If the link gets dropped from the index, it won't and this changes the "power" / "link juice" of every other page linked to it.
Second, if a spider passes through a link and doesn't index it, Google doesn't know its power. The power / link juice of a link is calculated AFTER it gets added to the index, not in the crawling phase. Spider crawling is a phase with a single purpose only: to discover links, not to index them, not to calculate their power.
Third, nofollow is just a tag implemented like 15 years ago and was supposed to help Google identify which sites are worth including in the index and which are not. It's a tag which has been invented in the very beginning of the web, however since it got abused to hell by webmasters, nowadays it's not worth at all. Whether you put a ******** or nofollow tag, this has nothing to do with Google's decision to spider a link, index it or calculate its power. Most SEO-s are obsessed with this tag, because they don't know better. This tag has absolutely no influence in the algorithms related to spidering, indexing or power calculation.
To recap, this is the process followed by Google (or any search engine for the matter):
1) Google bot spiders a link / page & checks for noindex tag. If it finds one, the link is not added to the index at all.
2) If the tag is not found, the link is run through an algorithm which checks if the link is "worthy" of inclusion in the index. If it's not, it's not added to the index.
3) If the link is worthy, it's added to the index.
4) Once in the index, algorithms calculating the power of links are re-run to take into account the newly added link(s).
5) Steps 1-4 are repeated again in an endless loop.