Engineering deep-dives, architecture tradeoffs, and build-in-public notes from behind the toolbox.
Open the black box of a 30-year loan: the amortization formula, why 87% of your first payment is interest, the full PITI stack, and the prepayment math that saves five figures — with free in-browser calculators.
One dollar, three prices. The 2026 cash stack — emergency fund in HYSA, dated goals in a CD ladder, the real-yield math after inflation, and the APY-vs-APR conversion banks hope you skip.
FIRE is a solvency equation, not a vibe: the Trinity Study’s real 4% rule, the Coast FIRE checkpoint you hit shockingly early, the account-stacking order that beats taxes, and guardrails for sequence risk.
CAC, LTV, and churn are one equation, not three KPIs. Fully-loaded CAC, margin-based LTV with churn in the denominator, the LTV/CAC ≥ 3 rule, payback period, and why 3% monthly churn is really 31% a year.
Zero-knowledge architecture in the browser: why GCM’s auth tag beats CBC, the IV-reuse rule that must never break, PBKDF2 parameters that stretch passwords properly, CSPRNG key generation, and JWT decoding vs verifying.
The clinician’s equation set — Mifflin-St Jeor BMR, honest TDEE activity tiers, the 7,700-kcal-per-kg deficit ledger, where 16:8 fasting actually helps, and how to fight adaptive thermogenesis.
LLM bills are multiplication: requests × tokens × price. Measure real p50/p95 token density, capture the 50–90% caching discount, split batch from interactive traffic, route easy work to small models, and stop over-optimizing embeddings.
A 50% markup is only a 33% margin — the gap is where profits leak. The formulas, price-from-target-margin, the real fee stack (2.9% + $0.30 is really 3.9% on a $30 order), freelance rates, and discount discipline.
Bag guidelines are written for intact, active, young dogs — yours probably isn’t. The RER = 70 × kg^0.75 baseline, MER life-stage multipliers from 3.0× puppies to 1.0× weight loss, the 10% treat rule, and the body condition score.
The waste factor separates a DIY weekend from a third trip to the store: 10% straight-lay flooring, 15–20% diagonal tile, 350–400 ft² per gallon per coat, batch-number matching, and the 10–20% budget contingency.
Lenders quote P&I and skip taxes, insurance, and PMI. The real numbers — full PITI, the five-figure lever of extra payments, the rent-vs-buy crossover year, and the minimum-payment trap — with free in-browser calculators.
A no-bullshit architecture retrospective — Next.js static export, lazy Service-Worker caching, 169+ JSON-LD pSEO schemas, and zero-CLS AdSense. The tradeoffs and the numbers.
Prefer doing over reading? Jump straight into the toolbox.