Total lipid content was measured gravimetrically or by the sum of lipid categories. You have to go back in the literature. Have a look at the paper attached.

Dear Annaduria,

Published papers used a wide range chemicals or natural products as standard in total lipid assay. But based on Knight et al or Ahlgren. you can use Pure Olive oil for this purpose.

pure olive oil (10 mgmL–1) in the 2:1 chloroform–

methanol solvent mixture as a stock standard. 

Make a search for vanillin; you can used it for measure total lipid from samples in a colorimetric way.