I implemented Evidence Explained, 2007, on which the FTM for Mac templates were based. [See FTM language in the template graphic below]. Mills then updated information and issued Evidence Explained, 2009. [note 1]
FamilySearch "adheres to the principles of 'Evidence Style' as presented in ... Evidence Explained ..." By observation, FamilySearch's "Citing this record" entries tend to follow more closely Mills v. 2009 (not 2007). (Then again, I haven't viewed every FamilySearch collection, much the underlying citations.)
More directly on point. Essential elements are below:
- Name of the database (you prefer this in quotes): "England, Births and Christenings, 1538-1975"
- Type of database: index (aka, "database," specifically not "database and digital images")
- Website (preferred in italics): FamilySearch
- URL: https://familysearch.org/pal:/MM9.1.1/NVLR-QFW (note: this is to the cited record, so you'll want this in the citation detail)
- Item of interest (entry in the database): William Long, 18 Jan 1833
- Source cited as the source (preferred in quotes; it is as represented to you): Berkeley, Gloucester, England, reference yr 1813-1826 p 108; FHL microfilm 855612
- Accessed Date/Year: 23 Jan 2013/2013
In FTM for Mac, I selected the template, "National Government Records > Databases > Database Online - Generic." (There are templates for United Kingdom records; I still chose the generic option.)

I entered generic information as follows

I hope it is obvious that you would substitute the terms "England, Births and Christenings, 1538-1975" (without the quotes, as the program adds those) for my "Title of Database."
The FTM for Mac default output from this template is below; it can be edited within the program.
"Title of Database", database, FamilySearch, Citation Detail.
Citation Text.
Given the different contraints, into the Citation Detail (FTM for Mac), I would type
William Long, 18 Jan 1833
(https://familysearch.org/pal:/MM9.1.1/NVLR-QFW : accessed 25 Jan
2013), citing "Berkeley, Gloucester, England, reference yr 1813-1826 p
108; FHL microfilm 855612."
There will be a little punctuation to clean up (comma out of place and extra period at the end), but all your basics are there.
Ratings
See also, Ancestry.com blog, "Rating a Source-Citation," 2008
- Original or Derivative [source]. It is a derivative, but it is an "index" or "finding aid" (ala, a transcription is a different kind of derivative).
- Clear or Marginal. This should really apply to the underlying record. In the context of a derivative such as this finding aid, the rating would be of limited use.
- Primary or Secondary [information]. Information is "a statement offered by a source" (Mills). This finding applies to the underlying record and not to the party extracting the record. In most cases a birth record is primary information about the birth of a person. (That doesn't mean it is without error.)
- Direct or Indirect [evidence]. Evidence is "information ... relevant to the problem" (Mills). A finding of "evidence" involves some logic and reasoning. For example, if there were several man, "William Long" whose births occurred about the same time, then this record alone might not answer your genealogical question. If the record does answer the genealogical question all by itself, then it is direct evidence. If the question is only answered when this record is considered with other records and information, then it is indirect evidence.
As an added personal note, ratings such as these reflect the genealogists constant work to seek out information from diversified and high quality sources.
note 1. I understood that little theory changed between the 2007 and 2009 editions (for the life of me, can not find that reference), but observe personally that the formatting changed considerably.