Audio CDs are written differently to disc compared to Data CDs, and that gives a information density advantage (2352 bytes/sector vs. 2048 bytes/sector), meaning 86 minutes of 44.1 kHz 16-bit stereo PCM audio take up 910 MB on your hard disk, but that is comparable to the same amount of CD-R sectors as 793 MB of data, hence fitting on 800 MiB discs.
If your audio files total more than that 910 MB (868 MiB), then perhaps some of those files are not 44.1 kHz 16-bit stereo. Toast, and most Audio-CD burning apps, will convert audio to that specification. There is no need for intermediate conversions. Standard Audio CDs are always uncompressed. (Some players may be able to play compressed files from a data CD, though.)
You should trust the “90 mins” for the disc as really having (close to) that capacity for Audio CD. However, 90 minute CD-R is non-standard, and Toast may or may not be able to use its full capacity: it may register as lesser duration and I think Toast has no ‘overburn’ capability (write data beyond registered limit).